diff options
| author | Stef Walter <stef@thewalter.net> | 2003-09-20 07:12:49 +0000 |
|---|---|---|
| committer | Stef Walter <stef@thewalter.net> | 2003-09-20 07:12:49 +0000 |
| commit | b49d8ebefe9b10c53a6a09ad564e22111b7b25c6 (patch) | |
| tree | 1d5dd4abc38170a7bc106dabbc59b915017222f0 /doc | |
| parent | 1cda9ebbd62916c7c2136722597a86c583e1ecf6 (diff) | |
Initial Import
Diffstat (limited to 'doc')
| -rw-r--r-- | doc/Makefile.am | 3 | ||||
| -rw-r--r-- | doc/language_v2.htm | 359 | ||||
| -rw-r--r-- | doc/pcre_man.txt | 1206 |
3 files changed, 1568 insertions, 0 deletions
diff --git a/doc/Makefile.am b/doc/Makefile.am new file mode 100644 index 0000000..04a654c --- /dev/null +++ b/doc/Makefile.am @@ -0,0 +1,3 @@ +EXTRA_DIST = language_v2.htm + + diff --git a/doc/language_v2.htm b/doc/language_v2.htm new file mode 100644 index 0000000..9e20493 --- /dev/null +++ b/doc/language_v2.htm @@ -0,0 +1,359 @@ +<!doctype html public "-//W3C//DTD HTML 4.0 Transitional//EN"> +<html> +<head> +<title> The rep language </title> +<style> + pre + { + margin-left: 3em; + } + h2 + { + margin-top: 3em; + } + h3 + { + margin-top: 2em; + font-weight: normal; + border-top: 1px solid black; + padding-left: 5px; + } + p + { + margin-left: 1em; + font-family: tahoma, verdana, arial, helvetica; + font-size: 9pt; + line-height: 12pt; + } +</style> +</head> + +<body bgcolor="#FFFFFF"> +<h1>The rep Language</h1> + +<p><a href="#syntax">Syntax</a><br> +<a href="#comments">Comments</a><br> +<a href="#commands">Commands</a><br> +<a href="#notes">Script Notes</a></p> + +<h2><a name="syntax">Syntax</a></h2> +<p>A rep script is made of various commands. The commands are detailed below, but here's a few basics:</p> +<p><strong>Options:</strong>Certain command can take various options. These are wrapped in parentheses, and separated by commas when multiple options are present.</p> +<p><strong>Text:</strong> Certain commands need a bit of text in order to do their thing. This could be a regular expresson, or perhaps replacement text. This text follows any options if present. The first character of the text is the quote character and is used to detect the end of the data block. If the quote character is used inside the text it must be escaped with a backslash. You can use any of these characters as a quote character:</p> + +<pre>"~`!@#$%^&*[]|'><./?+=-;:</pre> + +<p>The examples in this document will use a double quote <tt>(")</tt> as the quote character</p> + +<p>A general example of a rep command format would go something like this:</p> + +<pre>command (option, option) "data"</pre> + +<p>A command can extend to multiple lines. The following is valid:</p> + +<pre>command (options, +options) +"multiline +data"</pre> + +<p>A command can be followed by curly brackets. Generally this means that the result of the command applies to whatever commands are inside the curly brackets. If the command fails (for example a match that doesn't match) then the stuff inside the curly brackets isn't executed:</p> + +<pre>command +{ + more commands +}</pre> + +<h2><a name="comments">Comments</a></h2> + +<p>A comment starts in a # sign and extends to the end of the line. Comments are not valid inside data. For example:</p> + +<pre># Is a comment</pre> + +<h2><a name="commands">Commands</a></h2> + +<h3>match</h3> +<pre><b>syntax:</b> match (<i>not, once, find, tag, 0-9</i>) "<i>regexp</i>"</pre> +<p>Matches regular expression. A full study of regular expressions is outside the scope of this document. You can also match simple text, but you'll need to escape (with a backslash) any characters used by regular expressions. Those are: </p> + +<pre>.$%^*+?{}[]|()</pre> + +<p>Matches are not case sensitive unless specified with the 'case' option (see below).</p> + +<p>The statements inside the match are only executed if the match is successful. In addition the statements inside the match can only operate on the text that was matched. The following matches the word 'Hi There!' in a document and then matches the 'Hi' part.</p> + +<pre>match "Hi There!" +{ + # This only matches the above 'Hi' + match "Hi" + { + # Do something with 'Hi' + } +}</pre> + +<p>The match command can have several options:</p> + +<p><b>not</b>: Executes the contained statements if it doesn't match.</p> + +<pre>match (not) "can't find me"</pre> + +<p><b>once</b>: Makes sure this match only matches once in a document.</p> + +<pre>match (once) "<title>"</pre> + +<p><b>find</b>: Don't restrict statements inside the match to the text that was matched. This is useful for just verifying if something is there.</p> + +<pre>match (find) "check"</pre> + +<p><b>tag</b>: Makes a tag match. This is explained further below. + +<p><b>0-9</b>: Restricts the statements inside the match to the specified group (wrapped with paretheses) in the regular expression. 0 is the entire statement and 1 through 9 are numbered groups.</p> + +<pre>match (1) "Johnny (Smith)" +{ + # Now we can do something with 'Smith' +}</pre> + +<h3>replace</h3> + +<pre><b>syntax:</b> replace "<i>replace text</i>"</pre> + +<p>Replaces the matched text with new text. For example the following replaces 'Hello' with 'Yo' anywhere in the document:</p> + +<pre>match "Hello" +{ + replace "Yo" +}</pre> + +<p>You can include text groups (which were wrapped in parentheses) that were matched in the previous regular expression. These are specified by using a percent and the group number. %0 specifies all the matched text, and %1 - %9 are the numbered groups.</p> + +<p>For example the following replaces all <img> tags with <image> tags:</p> + +<pre>match "<img(.*?)>" +{ + replace "<image%1>" +}</pre> + +<p>After text has been replaced it is locked. It cannot be matched again.</p> + +<h3>else</h3> + +<pre><b>syntax:</b> else</pre> + +<p>Executes the contained statements if the above statements failed. For example the following executes if the match fails</p> + +<pre>match "Yo" +{ + # Do whatever we do with "Yo" +} +else +{ + # Didn't match. Do something else +}</pre> + +<h3>lock</h3> + +<pre><b>syntax:</b> lock</pre> + +<p>Locks text so it cannot be matched again. Useful to exclude portions of the document from replacements later on. The folling would lock all paragraphs in an HTML document.</p> + +<pre>match "<p>.*?</p>" +{ + lock +}</pre> + +<h3>loop</h3> + +<pre><b>syntax:</b> loop</pre> + +<p>Repeats the contained code until no more matches can be found. The following (dumb) example replaces all a's inside 'aardvark' with e's:</p> + +<pre>match "aardvark" +{ + loop + { + match "a" + { + replace "e" + } + } +}</pre> + +<p>Note that the entire document is actually wrapped in an invisible loop command. The entire document loops until no more matches can be found. However because of locking, the same text will generally not match more than once. In the above example that specific instance of 'aardvark' wouldn't match again after a single 'a' inside had been replaced or locked. Because of this the loop command comes in quite handy.</p> + +<h3>function</h3> + +<pre><b>syntax:</b> function <i>function_name</i></pre> + +<p>Defines a function which can be called with the call command. Function names are case sensitive, can be up to 40 characters long, and can consist of letters and the underscore.</p> + +<pre>function test +{ + # do whatever here +}</pre> + +<h3>call</h3> + +<pre><b>syntax:</b> call <i>function_name</i></pre> + +<p>Calls a function. The function must have been defined earlier in the document.</p> + +<pre>call test </pre> + +<p>The 'call' bit of the statement can be omitted, so the above can be shortened to:</p> + +<pre>test</pre> + +<p>For example the following function is used for multiple cases:</p> + +<pre>function(atoe) +{ + loop + { + match "a" + { + replace "e" + } + } +} + +match "aaron" +{ + atoe +} + +match "aardvark" +{ + atoe +}</pre> + +<h3>return</h3> + +<pre><b>syntax:</b>return (<i>number</i>)</pre> + +<p>When inside a function returns from that function to the code which called it. A function will normally return when it's end is reached, but <b>return</b> can be used to return earlier.</p> + +<p>You can also return a success code. This must be either 0 (for fail) or 1 (for success). In the code that called the function you can use <b>else</b> to take action on failure. </p> + +<h3>end</h3> + +<pre><b>syntax:</b> end</pre> + +<p>Stops the script at the current location. No more matches are done.</p> + +<h3>stop</h3> + +<pre><b>syntax:</b> stop "optional error message"</pre> + +<p>Stops the script as if an error had occurred. You can include an error message.</p> + +<h3>set</h3> + +<pre><b>syntax:</b> set <i>variable_name</i> "<i>value</i>"</pre> + +<p>Assigns the a value to a variable. The value can include backslash references to matched groups just like replace text can. Variables can be used both in later match statements, replaces or any text portion of a command. Variable names can consist of letters and the underscore. The name should alse be less than 40 characters long. Variables are used like so:</p> + +<pre>%<i>variable_name</i></pre> + +<p>The following example gets a heading from an HTML document and sets the title to it:</p> + +<pre>match "<h1>(.*?)<h1>" +{ + setv heading "%1" +} + +match "<head>" +{ + replace "<head><title>%heading</title>" +}</pre> + +<p>An example of using a variable as a match would be the following. The title is matched, and then searched for in the document. If it's found it's made bold:</p> + +<pre>match (once) "<title>(.*?)</title>" +{ + set title "%1" +} + +match "%title" +{ + replace "<b>%0</b> +}</pre> + +<p>If a variable is used whose value hasn't been set yet, it's value is blank. You may also use environment variables in your document. This is how you'd pass values to your script from outside.</p> + +<h3>add</h3> + +<pre><b>syntax:</b> add <i>variable_name</i> "<i>value</i>"</pre> + +<p>Similar to <b>set</b> but insteads makes a variable array and adds the value to it. That means multiple values can be set to a variable. This is only useful during matches where any one of the variable values will match. If a multiple variable is used anywhere else an error will result.</p> + +<p>The syntax for using a multiple variable is:</p> + +<pre>%(<i>variable_name</i>)</pre> + +<p>If used without the parentheses the variable will act like a normal variable and the first of it's values will be used.</p> + +<p>The following example matches any HTML table element:</p> + +<pre>set telement "table" +add telement "td" +add telement "tr" +add telement "tbody" +add telement "thead" + +match "%(telement)" +{ + # Do whatever +}</pre> + +<h3>clr</h3> + +<pre><b>syntax:</b> clr <i>variable_name</i></pre> + +<p>Clears a variable of it's value.</p> + +<h3>options</h3> + +<pre><b>syntax:</b> options(case, line)</pre> + +<p>Sets various options for your script. These options will apply only inside the set of curly braces that they are set. To set global options put the <b>options</b> command at the top of your file. The options are:</p> + +<p><b>case:</b> Makes the matches case sensitive.</p> + +<p><b>line:</b> Restricts matches to one line.</p> + +<p><i>delimiter:</i> Sets the delimiter for the data portions of commands. By default the double-quote is used, but if you need to match this often, then you can change the delimiter to something else.</p> + +<p>The following makes matches case sensitive:</p> + +<pre>options(case)</pre> + +<h2><a name="notes">Script Notes</a></h2> + +<p>First of all you'll need to know regular expressions to get anything useful out of rep scripting. The regular expressions used in rep are PCRE (Perl Compatible Regular Expressions). They're not explained here but you can get tons of info online for them.</p> + +<p>Once your script gets a little more than just two or three commands, it's important to understand how it gets run:</p> + +<p>The entire script is run over and over again in a loop until no more matches are made. Although it's difficult, it is possible to throw your script in an endless loop. If this is the case then execution stops after a million loops.</p> + +<p>Portions of the document that have been locked or replaced cannot be matched again. Also if a portion of text to be matched has been locked it cannot be matched. Keep this in mind when making your scripts. If you wish to match and replace multiple items inside another match, you'll need to use a loop command to do so.</p> + +<p>The rep processor can be used in a buffered mode where only a portion of the document is operated on at one time. This greatly increases the speed of the processor. But you have to be careful that any matches you make will fit inside that buffer. In many cases (for example matching the entire <body> tag of an HTML document, you won't be able to use buffered mode reliably.)</p> + +<h3>Tag Matches</h3> +<p>Tag matches match a starting and closing tag of your choice. They also take into consideration that there may be other tags inside that could match. The opening and closing tag are separated by an equal sign ('<tt>=</tt>').</p> + +<p>For example the following would match a set of <tt><div></tt> tags in an HTML document. It would pair up the correct set of <tt><div></tt> tags even if there were other contained tags that could match:</p> + +<pre>match (tag) "<div>=</div>"</pre> + +<p>The regular expression groups are handled differently when using tag matching:</p> + +<p><b>0</b>: This is the entire match as usual.<br> +<b>1</b>: The text of the opening tag.<br> +<b>2</b>: The contained text.<br> +<b>3</b>: The text of the closing tag.</p> + + + +</body> +</html> diff --git a/doc/pcre_man.txt b/doc/pcre_man.txt new file mode 100644 index 0000000..e3b1c7d --- /dev/null +++ b/doc/pcre_man.txt @@ -0,0 +1,1206 @@ +NAME + pcre - Perl-compatible regular expressions. + + +REGULAR EXPRESSION DETAILS + The syntax and semantics of the regular expressions sup- + ported by PCRE are described below. Regular expressions are + also described in the Perl documentation and in a number of + other books, some of which have copious examples. Jeffrey + Friedl's "Mastering Regular Expressions", published by + O'Reilly (ISBN 1-56592-257), covers them in great detail. + + The description here is intended as reference documentation. + The basic operation of PCRE is on strings of bytes. However, + there is the beginnings of some support for UTF-8 character + strings. To use this support you must configure PCRE to + include it, and then call pcre_compile() with the PCRE_UTF8 + option. How this affects the pattern matching is described + in the final section of this document. + + A regular expression is a pattern that is matched against a + subject string from left to right. Most characters stand for + themselves in a pattern, and match the corresponding charac- + ters in the subject. As a trivial example, the pattern + + The quick brown fox + + matches a portion of a subject string that is identical to + itself. The power of regular expressions comes from the + ability to include alternatives and repetitions in the pat- + tern. These are encoded in the pattern by the use of meta- + characters, which do not stand for themselves but instead + are interpreted in some special way. + + There are two different sets of meta-characters: those that + are recognized anywhere in the pattern except within square + brackets, and those that are recognized in square brackets. + Outside square brackets, the meta-characters are as follows: + + \ general escape character with several uses + ^ assert start of subject (or line, in multiline + mode) + $ assert end of subject (or line, in multiline mode) + . match any character except newline (by default) + [ start character class definition + | start of alternative branch + ( start subpattern + ) end subpattern + ? extends the meaning of ( + also 0 or 1 quantifier + also quantifier minimizer + * 0 or more quantifier + + 1 or more quantifier + { start min/max quantifier + + Part of a pattern that is in square brackets is called a + "character class". In a character class the only meta- + characters are: + + \ general escape character + ^ negate the class, but only if the first character + - indicates character range + ] terminates the character class + + The following sections describe the use of each of the + meta-characters. + + + +BACKSLASH + The backslash character has several uses. Firstly, if it is + followed by a non-alphameric character, it takes away any + special meaning that character may have. This use of + + backslash as an escape character applies both inside and + outside character classes. + + For example, if you want to match a "*" character, you write + "\*" in the pattern. This applies whether or not the follow- + ing character would otherwise be interpreted as a meta- + character, so it is always safe to precede a non-alphameric + with "\" to specify that it stands for itself. In particu- + lar, if you want to match a backslash, you write "\\". + + If a pattern is compiled with the PCRE_EXTENDED option, whi- + tespace in the pattern (other than in a character class) and + characters between a "#" outside a character class and the + next newline character are ignored. An escaping backslash + can be used to include a whitespace or "#" character as part + of the pattern. + + A second use of backslash provides a way of encoding non- + printing characters in patterns in a visible manner. There + is no restriction on the appearance of non-printing charac- + ters, apart from the binary zero that terminates a pattern, + but when a pattern is being prepared by text editing, it is + usually easier to use one of the following escape sequences + than the binary character it represents: + + \a alarm, that is, the BEL character (hex 07) + \cx "control-x", where x is any character + \e escape (hex 1B) + \f formfeed (hex 0C) + \n newline (hex 0A) + \r carriage return (hex 0D) + \t tab (hex 09) + \xhh character with hex code hh + \ddd character with octal code ddd, or backreference + + The precise effect of "\cx" is as follows: if "x" is a lower + case letter, it is converted to upper case. Then bit 6 of + the character (hex 40) is inverted. Thus "\cz" becomes hex + 1A, but "\c{" becomes hex 3B, while "\c;" becomes hex 7B. + + After "\x", up to two hexadecimal digits are read (letters + can be in upper or lower case). + + After "\0" up to two further octal digits are read. In both + cases, if there are fewer than two digits, just those that + are present are used. Thus the sequence "\0\x\07" specifies + two binary zeros followed by a BEL character. Make sure you + supply two digits after the initial zero if the character + that follows is itself an octal digit. + + The handling of a backslash followed by a digit other than 0 + is complicated. Outside a character class, PCRE reads it + and any following digits as a decimal number. If the number + is less than 10, or if there have been at least that many + previous capturing left parentheses in the expression, the + entire sequence is taken as a back reference. A description + of how this works is given later, following the discussion + of parenthesized subpatterns. + + Inside a character class, or if the decimal number is + greater than 9 and there have not been that many capturing + subpatterns, PCRE re-reads up to three octal digits follow- + ing the backslash, and generates a single byte from the + least significant 8 bits of the value. Any subsequent digits + stand for themselves. For example: + + \040 is another way of writing a space + \40 is the same, provided there are fewer than 40 + previous capturing subpatterns + \7 is always a back reference + \11 might be a back reference, or another way of + writing a tab + \011 is always a tab + \0113 is a tab followed by the character "3" + \113 is the character with octal code 113 (since there + can be no more than 99 back references) + \377 is a byte consisting entirely of 1 bits + \81 is either a back reference, or a binary zero + followed by the two characters "8" and "1" + + Note that octal values of 100 or greater must not be intro- + duced by a leading zero, because no more than three octal + digits are ever read. + + All the sequences that define a single byte value can be + used both inside and outside character classes. In addition, + inside a character class, the sequence "\b" is interpreted + as the backspace character (hex 08). Outside a character + class it has a different meaning (see below). + + The third use of backslash is for specifying generic charac- + ter types: + + \d any decimal digit + \D any character that is not a decimal digit + \s any whitespace character + \S any character that is not a whitespace character + \w any "word" character + \W any "non-word" character + + Each pair of escape sequences partitions the complete set of + characters into two disjoint sets. Any given character + matches one, and only one, of each pair. + + A "word" character is any letter or digit or the underscore + character, that is, any character which can be part of a + Perl "word". The definition of letters and digits is con- + trolled by PCRE's character tables, and may vary if locale- + specific matching is taking place (see "Locale support" + above). For example, in the "fr" (French) locale, some char- + acter codes greater than 128 are used for accented letters, + and these are matched by \w. + + These character type sequences can appear both inside and + outside character classes. They each match one character of + the appropriate type. If the current matching point is at + the end of the subject string, all of them fail, since there + is no character to match. + + The fourth use of backslash is for certain simple asser- + tions. An assertion specifies a condition that has to be met + at a particular point in a match, without consuming any + characters from the subject string. The use of subpatterns + for more complicated assertions is described below. The + backslashed assertions are + + \b word boundary + \B not a word boundary + \A start of subject (independent of multiline mode) + \Z end of subject or newline at end (independent of + multiline mode) + \z end of subject (independent of multiline mode) + + These assertions may not appear in character classes (but + note that "\b" has a different meaning, namely the backspace + character, inside a character class). + + A word boundary is a position in the subject string where + the current character and the previous character do not both + match \w or \W (i.e. one matches \w and the other matches + \W), or the start or end of the string if the first or last + character matches \w, respectively. + + The \A, \Z, and \z assertions differ from the traditional + circumflex and dollar (described below) in that they only + ever match at the very start and end of the subject string, + whatever options are set. They are not affected by the + PCRE_NOTBOL or PCRE_NOTEOL options. If the startoffset argu- + ment of pcre_exec() is non-zero, \A can never match. The + difference between \Z and \z is that \Z matches before a + newline that is the last character of the string as well as + at the end of the string, whereas \z matches only at the + end. + + + +CIRCUMFLEX AND DOLLAR + Outside a character class, in the default matching mode, the + circumflex character is an assertion which is true only if + the current matching point is at the start of the subject + string. If the startoffset argument of pcre_exec() is non- + zero, circumflex can never match. Inside a character class, + circumflex has an entirely different meaning (see below). + + Circumflex need not be the first character of the pattern if + a number of alternatives are involved, but it should be the + first thing in each alternative in which it appears if the + pattern is ever to match that branch. If all possible alter- + natives start with a circumflex, that is, if the pattern is + constrained to match only at the start of the subject, it is + said to be an "anchored" pattern. (There are also other con- + structs that can cause a pattern to be anchored.) + + A dollar character is an assertion which is true only if the + current matching point is at the end of the subject string, + or immediately before a newline character that is the last + character in the string (by default). Dollar need not be the + last character of the pattern if a number of alternatives + are involved, but it should be the last item in any branch + in which it appears. Dollar has no special meaning in a + character class. + + The meaning of dollar can be changed so that it matches only + at the very end of the string, by setting the + PCRE_DOLLAR_ENDONLY option at compile or matching time. This + does not affect the \Z assertion. + + The meanings of the circumflex and dollar characters are + changed if the PCRE_MULTILINE option is set. When this is + the case, they match immediately after and immediately + before an internal "\n" character, respectively, in addition + to matching at the start and end of the subject string. For + example, the pattern /^abc$/ matches the subject string + "def\nabc" in multiline mode, but not otherwise. Conse- + quently, patterns that are anchored in single line mode + because all branches start with "^" are not anchored in mul- + tiline mode, and a match for circumflex is possible when the + startoffset argument of pcre_exec() is non-zero. The + PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is + set. + + Note that the sequences \A, \Z, and \z can be used to match + the start and end of the subject in both modes, and if all + branches of a pattern start with \A it is always anchored, + whether PCRE_MULTILINE is set or not. + + + +FULL STOP (PERIOD, DOT) + Outside a character class, a dot in the pattern matches any + one character in the subject, including a non-printing char- + acter, but not (by default) newline. If the PCRE_DOTALL + option is set, dots match newlines as well. The handling of + dot is entirely independent of the handling of circumflex + and dollar, the only relationship being that they both + involve newline characters. Dot has no special meaning in a + character class. + + + +SQUARE BRACKETS + An opening square bracket introduces a character class, ter- + minated by a closing square bracket. A closing square + bracket on its own is not special. If a closing square + bracket is required as a member of the class, it should be + the first data character in the class (after an initial cir- + cumflex, if present) or escaped with a backslash. + + A character class matches a single character in the subject; + the character must be in the set of characters defined by + the class, unless the first character in the class is a cir- + cumflex, in which case the subject character must not be in + the set defined by the class. If a circumflex is actually + required as a member of the class, ensure it is not the + first character, or escape it with a backslash. + + For example, the character class [aeiou] matches any lower + case vowel, while [^aeiou] matches any character that is not + a lower case vowel. Note that a circumflex is just a con- + venient notation for specifying the characters which are in + the class by enumerating those that are not. It is not an + assertion: it still consumes a character from the subject + string, and fails if the current pointer is at the end of + the string. + + When caseless matching is set, any letters in a class + represent both their upper case and lower case versions, so + for example, a caseless [aeiou] matches "A" as well as "a", + and a caseless [^aeiou] does not match "A", whereas a case- + ful version would. + + The newline character is never treated in any special way in + character classes, whatever the setting of the PCRE_DOTALL + or PCRE_MULTILINE options is. A class such as [^a] will + always match a newline. + + The minus (hyphen) character can be used to specify a range + of characters in a character class. For example, [d-m] + matches any letter between d and m, inclusive. If a minus + character is required in a class, it must be escaped with a + backslash or appear in a position where it cannot be inter- + preted as indicating a range, typically as the first or last + character in the class. + + It is not possible to have the literal character "]" as the + end character of a range. A pattern such as [W-]46] is + interpreted as a class of two characters ("W" and "-") fol- + lowed by a literal string "46]", so it would match "W46]" or + "-46]". However, if the "]" is escaped with a backslash it + is interpreted as the end of range, so [W-\]46] is inter- + preted as a single class containing a range followed by two + separate characters. The octal or hexadecimal representation + of "]" can also be used to end a range. + + Ranges operate in ASCII collating sequence. They can also be + used for characters specified numerically, for example + [\000-\037]. If a range that includes letters is used when + caseless matching is set, it matches the letters in either + case. For example, [W-c] is equivalent to [][\^_`wxyzabc], + matched caselessly, and if character tables for the "fr" + locale are in use, [\xc8-\xcb] matches accented E characters + in both cases. + + The character types \d, \D, \s, \S, \w, and \W may also + appear in a character class, and add the characters that + they match to the class. For example, [\dABCDEF] matches any + hexadecimal digit. A circumflex can conveniently be used + with the upper case character types to specify a more res- + tricted set of characters than the matching lower case type. + For example, the class [^\W_] matches any letter or digit, + but not underscore. + + All non-alphameric characters other than \, -, ^ (at the + start) and the terminating ] are non-special in character + classes, but it does no harm if they are escaped. + + + +POSIX CHARACTER CLASSES + Perl 5.6 (not yet released at the time of writing) is going + to support the POSIX notation for character classes, which + uses names enclosed by [: and :] within the enclosing + square brackets. PCRE supports this notation. For example, + + [01[:alpha:]%] + + matches "0", "1", any alphabetic character, or "%". The sup- + ported class names are + + alnum letters and digits + alpha letters + ascii character codes 0 - 127 + cntrl control characters + digit decimal digits (same as \d) + graph printing characters, excluding space + lower lower case letters + print printing characters, including space + punct printing characters, excluding letters and digits + space white space (same as \s) + upper upper case letters + word "word" characters (same as \w) + xdigit hexadecimal digits + + The names "ascii" and "word" are Perl extensions. Another + Perl extension is negation, which is indicated by a ^ char- + acter after the colon. For example, + + [12[:^digit:]] + + matches "1", "2", or any non-digit. PCRE (and Perl) also + recognize the POSIX syntax [.ch.] and [=ch=] where "ch" is a + "collating element", but these are not supported, and an + error is given if they are encountered. + + + +VERTICAL BAR + Vertical bar characters are used to separate alternative + patterns. For example, the pattern + + gilbert|sullivan + + matches either "gilbert" or "sullivan". Any number of alter- + natives may appear, and an empty alternative is permitted + (matching the empty string). The matching process tries + each alternative in turn, from left to right, and the first + one that succeeds is used. If the alternatives are within a + subpattern (defined below), "succeeds" means matching the + rest of the main pattern as well as the alternative in the + subpattern. + + + +INTERNAL OPTION SETTING + The settings of PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, + and PCRE_EXTENDED can be changed from within the pattern by + a sequence of Perl option letters enclosed between "(?" and + ")". The option letters are + + i for PCRE_CASELESS + m for PCRE_MULTILINE + s for PCRE_DOTALL + x for PCRE_EXTENDED + + For example, (?im) sets caseless, multiline matching. It is + also possible to unset these options by preceding the letter + with a hyphen, and a combined setting and unsetting such as + (?im-sx), which sets PCRE_CASELESS and PCRE_MULTILINE while + unsetting PCRE_DOTALL and PCRE_EXTENDED, is also permitted. + If a letter appears both before and after the hyphen, the + option is unset. + + The scope of these option changes depends on where in the + pattern the setting occurs. For settings that are outside + any subpattern (defined below), the effect is the same as if + the options were set or unset at the start of matching. The + following patterns all behave in exactly the same way: + + (?i)abc + a(?i)bc + ab(?i)c + abc(?i) + + which in turn is the same as compiling the pattern abc with + PCRE_CASELESS set. In other words, such "top level" set- + tings apply to the whole pattern (unless there are other + changes inside subpatterns). If there is more than one set- + ting of the same option at top level, the rightmost setting + is used. + + If an option change occurs inside a subpattern, the effect + is different. This is a change of behaviour in Perl 5.005. + An option change inside a subpattern affects only that part + of the subpattern that follows it, so + + (a(?i)b)c + + matches abc and aBc and no other strings (assuming + PCRE_CASELESS is not used). By this means, options can be + made to have different settings in different parts of the + pattern. Any changes made in one alternative do carry on + into subsequent branches within the same subpattern. For + example, + + (a(?i)b|c) + + matches "ab", "aB", "c", and "C", even though when matching + "C" the first branch is abandoned before the option setting. + This is because the effects of option settings happen at + compile time. There would be some very weird behaviour oth- + erwise. + + The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can + be changed in the same way as the Perl-compatible options by + using the characters U and X respectively. The (?X) flag + setting is special in that it must always occur earlier in + the pattern than any of the additional features it turns on, + even when it is at top level. It is best put at the start. + + + +SUBPATTERNS + Subpatterns are delimited by parentheses (round brackets), + which can be nested. Marking part of a pattern as a subpat- + tern does two things: + + 1. It localizes a set of alternatives. For example, the pat- + tern + + cat(aract|erpillar|) + + matches one of the words "cat", "cataract", or "caterpil- + lar". Without the parentheses, it would match "cataract", + "erpillar" or the empty string. + + 2. It sets up the subpattern as a capturing subpattern (as + defined above). When the whole pattern matches, that por- + tion of the subject string that matched the subpattern is + passed back to the caller via the ovector argument of + pcre_exec(). Opening parentheses are counted from left to + right (starting from 1) to obtain the numbers of the captur- + ing subpatterns. + + For example, if the string "the red king" is matched against + the pattern + + the ((red|white) (king|queen)) + + the captured substrings are "red king", "red", and "king", + and are numbered 1, 2, and 3, respectively. + + The fact that plain parentheses fulfil two functions is not + always helpful. There are often times when a grouping sub- + pattern is required without a capturing requirement. If an + opening parenthesis is followed by "?:", the subpattern does + not do any capturing, and is not counted when computing the + number of any subsequent capturing subpatterns. For example, + if the string "the white queen" is matched against the pat- + tern + + the ((?:red|white) (king|queen)) + + the captured substrings are "white queen" and "queen", and + are numbered 1 and 2. The maximum number of captured sub- + strings is 99, and the maximum number of all subpatterns, + both capturing and non-capturing, is 200. + + As a convenient shorthand, if any option settings are + required at the start of a non-capturing subpattern, the + option letters may appear between the "?" and the ":". Thus + the two patterns + + (?i:saturday|sunday) + (?:(?i)saturday|sunday) + + match exactly the same set of strings. Because alternative + branches are tried from left to right, and options are not + reset until the end of the subpattern is reached, an option + setting in one branch does affect subsequent branches, so + the above patterns match "SUNDAY" as well as "Saturday". + + + +REPETITION + Repetition is specified by quantifiers, which can follow any + of the following items: + + a single character, possibly escaped + the . metacharacter + a character class + a back reference (see next section) + a parenthesized subpattern (unless it is an assertion - + see below) + + The general repetition quantifier specifies a minimum and + maximum number of permitted matches, by giving the two + numbers in curly brackets (braces), separated by a comma. + The numbers must be less than 65536, and the first must be + less than or equal to the second. For example: + + z{2,4} + + matches "zz", "zzz", or "zzzz". A closing brace on its own + is not a special character. If the second number is omitted, + but the comma is present, there is no upper limit; if the + second number and the comma are both omitted, the quantifier + specifies an exact number of required matches. Thus + + [aeiou]{3,} + + matches at least 3 successive vowels, but may match many + more, while + + \d{8} + + matches exactly 8 digits. An opening curly bracket that + appears in a position where a quantifier is not allowed, or + one that does not match the syntax of a quantifier, is taken + as a literal character. For example, {,6} is not a quantif- + ier, but a literal string of four characters. + The quantifier {0} is permitted, causing the expression to + behave as if the previous item and the quantifier were not + present. + + For convenience (and historical compatibility) the three + most common quantifiers have single-character abbreviations: + + * is equivalent to {0,} + + is equivalent to {1,} + ? is equivalent to {0,1} + + It is possible to construct infinite loops by following a + subpattern that can match no characters with a quantifier + that has no upper limit, for example: + + (a?)* + + Earlier versions of Perl and PCRE used to give an error at + compile time for such patterns. However, because there are + cases where this can be useful, such patterns are now + accepted, but if any repetition of the subpattern does in + fact match no characters, the loop is forcibly broken. + + By default, the quantifiers are "greedy", that is, they + match as much as possible (up to the maximum number of per- + mitted times), without causing the rest of the pattern to + fail. The classic example of where this gives problems is in + trying to match comments in C programs. These appear between + the sequences /* and */ and within the sequence, individual + * and / characters may appear. An attempt to match C com- + ments by applying the pattern + + /\*.*\*/ + + to the string + + /* first command */ not comment /* second comment */ + + fails, because it matches the entire string owing to the + greediness of the .* item. + + However, if a quantifier is followed by a question mark, it + ceases to be greedy, and instead matches the minimum number + of times possible, so the pattern + + /\*.*?\*/ + + does the right thing with the C comments. The meaning of the + various quantifiers is not otherwise changed, just the pre- + ferred number of matches. Do not confuse this use of ques- + tion mark with its use as a quantifier in its own right. + Because it has two uses, it can sometimes appear doubled, as + in + + \d??\d + + which matches one digit by preference, but can match two if + that is the only way the rest of the pattern matches. + + If the PCRE_UNGREEDY option is set (an option which is not + available in Perl), the quantifiers are not greedy by + default, but individual ones can be made greedy by following + them with a question mark. In other words, it inverts the + default behaviour. + + When a parenthesized subpattern is quantified with a minimum + repeat count that is greater than 1 or with a limited max- + imum, more store is required for the compiled pattern, in + proportion to the size of the minimum or maximum. + + If a pattern starts with .* or .{0,} and the PCRE_DOTALL + option (equivalent to Perl's /s) is set, thus allowing the . + to match newlines, the pattern is implicitly anchored, + because whatever follows will be tried against every charac- + ter position in the subject string, so there is no point in + retrying the overall match at any position after the first. + PCRE treats such a pattern as though it were preceded by \A. + In cases where it is known that the subject string contains + no newlines, it is worth setting PCRE_DOTALL when the pat- + tern begins with .* in order to obtain this optimization, or + alternatively using ^ to indicate anchoring explicitly. + + When a capturing subpattern is repeated, the value captured + is the substring that matched the final iteration. For exam- + ple, after + + (tweedle[dume]{3}\s*)+ + + has matched "tweedledum tweedledee" the value of the cap- + tured substring is "tweedledee". However, if there are + nested capturing subpatterns, the corresponding captured + values may have been set in previous iterations. For exam- + ple, after + + /(a|(b))+/ + + matches "aba" the value of the second captured substring is + "b". + + + +BACK REFERENCES + Outside a character class, a backslash followed by a digit + greater than 0 (and possibly further digits) is a back + reference to a capturing subpattern earlier (i.e. to its + left) in the pattern, provided there have been that many + previous capturing left parentheses. + + However, if the decimal number following the backslash is + less than 10, it is always taken as a back reference, and + causes an error only if there are not that many capturing + left parentheses in the entire pattern. In other words, the + parentheses that are referenced need not be to the left of + the reference for numbers less than 10. See the section + entitled "Backslash" above for further details of the han- + dling of digits following a backslash. + + A back reference matches whatever actually matched the cap- + turing subpattern in the current subject string, rather than + anything matching the subpattern itself. So the pattern + + (sens|respons)e and \1ibility + + matches "sense and sensibility" and "response and responsi- + bility", but not "sense and responsibility". If caseful + matching is in force at the time of the back reference, the + case of letters is relevant. For example, + + ((?i)rah)\s+\1 + + matches "rah rah" and "RAH RAH", but not "RAH rah", even + though the original capturing subpattern is matched case- + lessly. + + There may be more than one back reference to the same sub- + pattern. If a subpattern has not actually been used in a + particular match, any back references to it always fail. For + example, the pattern + + (a|(bc))\2 + + always fails if it starts to match "a" rather than "bc". + Because there may be up to 99 back references, all digits + following the backslash are taken as part of a potential + back reference number. If the pattern continues with a digit + character, some delimiter must be used to terminate the back + reference. If the PCRE_EXTENDED option is set, this can be + whitespace. Otherwise an empty comment can be used. + + A back reference that occurs inside the parentheses to which + it refers fails when the subpattern is first used, so, for + example, (a\1) never matches. However, such references can + be useful inside repeated subpatterns. For example, the pat- + tern + + (a|b\1)+ + + matches any number of "a"s and also "aba", "ababbaa" etc. At + each iteration of the subpattern, the back reference matches + the character string corresponding to the previous itera- + tion. In order for this to work, the pattern must be such + that the first iteration does not need to match the back + reference. This can be done using alternation, as in the + example above, or by a quantifier with a minimum of zero. + + + +ASSERTIONS + An assertion is a test on the characters following or + preceding the current matching point that does not actually + consume any characters. The simple assertions coded as \b, + \B, \A, \Z, \z, ^ and $ are described above. More compli- + cated assertions are coded as subpatterns. There are two + kinds: those that look ahead of the current position in the + subject string, and those that look behind it. + + An assertion subpattern is matched in the normal way, except + that it does not cause the current matching position to be + changed. Lookahead assertions start with (?= for positive + assertions and (?! for negative assertions. For example, + + \w+(?=;) + + matches a word followed by a semicolon, but does not include + the semicolon in the match, and + + foo(?!bar) + + matches any occurrence of "foo" that is not followed by + "bar". Note that the apparently similar pattern + + (?!foo)bar + + does not find an occurrence of "bar" that is preceded by + something other than "foo"; it finds any occurrence of "bar" + whatsoever, because the assertion (?!foo) is always true + when the next three characters are "bar". A lookbehind + assertion is needed to achieve this effect. + + Lookbehind assertions start with (?<= for positive asser- + tions and (?<! for negative assertions. For example, + + (?<!foo)bar + + does find an occurrence of "bar" that is not preceded by + "foo". The contents of a lookbehind assertion are restricted + such that all the strings it matches must have a fixed + length. However, if there are several alternatives, they do + not all have to have the same fixed length. Thus + + (?<=bullock|donkey) + + is permitted, but + + (?<!dogs?|cats?) + + causes an error at compile time. Branches that match dif- + ferent length strings are permitted only at the top level of + a lookbehind assertion. This is an extension compared with + Perl 5.005, which requires all branches to match the same + length of string. An assertion such as + + (?<=ab(c|de)) + + is not permitted, because its single top-level branch can + match two different lengths, but it is acceptable if rewrit- + ten to use two top-level branches: + + (?<=abc|abde) + + The implementation of lookbehind assertions is, for each + alternative, to temporarily move the current position back + by the fixed width and then try to match. If there are + insufficient characters before the current position, the + match is deemed to fail. Lookbehinds in conjunction with + once-only subpatterns can be particularly useful for match- + ing at the ends of strings; an example is given at the end + of the section on once-only subpatterns. + + Several assertions (of any sort) may occur in succession. + For example, + + (?<=\d{3})(?<!999)foo + + matches "foo" preceded by three digits that are not "999". + Notice that each of the assertions is applied independently + at the same point in the subject string. First there is a + check that the previous three characters are all digits, and + then there is a check that the same three characters are not + "999". This pattern does not match "foo" preceded by six + characters, the first of which are digits and the last three + of which are not "999". For example, it doesn't match + "123abcfoo". A pattern to do that is + + (?<=\d{3}...)(?<!999)foo + + This time the first assertion looks at the preceding six + characters, checking that the first three are digits, and + then the second assertion checks that the preceding three + characters are not "999". + + Assertions can be nested in any combination. For example, + + (?<=(?<!foo)bar)baz + + matches an occurrence of "baz" that is preceded by "bar" + which in turn is not preceded by "foo", while + + (?<=\d{3}(?!999)...)foo + + is another pattern which matches "foo" preceded by three + digits and any three characters that are not "999". + + Assertion subpatterns are not capturing subpatterns, and may + not be repeated, because it makes no sense to assert the + same thing several times. If any kind of assertion contains + capturing subpatterns within it, these are counted for the + purposes of numbering the capturing subpatterns in the whole + pattern. However, substring capturing is carried out only + for positive assertions, because it does not make sense for + negative assertions. + + Assertions count towards the maximum of 200 parenthesized + subpatterns. + + + +ONCE-ONLY SUBPATTERNS + With both maximizing and minimizing repetition, failure of + what follows normally causes the repeated item to be re- + evaluated to see if a different number of repeats allows the + rest of the pattern to match. Sometimes it is useful to + prevent this, either to change the nature of the match, or + to cause it fail earlier than it otherwise might, when the + author of the pattern knows there is no point in carrying + on. + + Consider, for example, the pattern \d+foo when applied to + the subject line + + 123456bar + + After matching all 6 digits and then failing to match "foo", + the normal action of the matcher is to try again with only 5 + digits matching the \d+ item, and then with 4, and so on, + before ultimately failing. Once-only subpatterns provide the + means for specifying that once a portion of the pattern has + matched, it is not to be re-evaluated in this way, so the + matcher would give up immediately on failing to match "foo" + the first time. The notation is another kind of special + parenthesis, starting with (?> as in this example: + + (?>\d+)bar + + This kind of parenthesis "locks up" the part of the pattern + it contains once it has matched, and a failure further into + the pattern is prevented from backtracking into it. Back- + tracking past it to previous items, however, works as nor- + mal. + + An alternative description is that a subpattern of this type + matches the string of characters that an identical stan- + dalone pattern would match, if anchored at the current point + in the subject string. + + Once-only subpatterns are not capturing subpatterns. Simple + cases such as the above example can be thought of as a max- + imizing repeat that must swallow everything it can. So, + while both \d+ and \d+? are prepared to adjust the number of + digits they match in order to make the rest of the pattern + match, (?>\d+) can only match an entire sequence of digits. + + This construction can of course contain arbitrarily compli- + cated subpatterns, and it can be nested. + + Once-only subpatterns can be used in conjunction with look- + behind assertions to specify efficient matching at the end + of the subject string. Consider a simple pattern such as + + abcd$ + + when applied to a long string which does not match. Because + matching proceeds from left to right, PCRE will look for + each "a" in the subject and then see if what follows matches + the rest of the pattern. If the pattern is specified as + + ^.*abcd$ + + the initial .* matches the entire string at first, but when + this fails (because there is no following "a"), it back- + tracks to match all but the last character, then all but the + last two characters, and so on. Once again the search for + "a" covers the entire string, from right to left, so we are + no better off. However, if the pattern is written as + + ^(?>.*)(?<=abcd) + + there can be no backtracking for the .* item; it can match + only the entire string. The subsequent lookbehind assertion + does a single test on the last four characters. If it fails, + the match fails immediately. For long strings, this approach + makes a significant difference to the processing time. + + When a pattern contains an unlimited repeat inside a subpat- + tern that can itself be repeated an unlimited number of + times, the use of a once-only subpattern is the only way to + avoid some failing matches taking a very long time indeed. + The pattern + + (\D+|<\d+>)*[!?] + + matches an unlimited number of substrings that either con- + sist of non-digits, or digits enclosed in <>, followed by + either ! or ?. When it matches, it runs quickly. However, if + it is applied to + + aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa + + it takes a long time before reporting failure. This is + because the string can be divided between the two repeats in + a large number of ways, and all have to be tried. (The exam- + ple used [!?] rather than a single character at the end, + because both PCRE and Perl have an optimization that allows + for fast failure when a single character is used. They + remember the last single character that is required for a + match, and fail early if it is not present in the string.) + If the pattern is changed to + + ((?>\D+)|<\d+>)*[!?] + + sequences of non-digits cannot be broken, and failure hap- + pens quickly. + + + +CONDITIONAL SUBPATTERNS + It is possible to cause the matching process to obey a sub- + pattern conditionally or to choose between two alternative + subpatterns, depending on the result of an assertion, or + whether a previous capturing subpattern matched or not. The + two possible forms of conditional subpattern are + + (?(condition)yes-pattern) + (?(condition)yes-pattern|no-pattern) + + If the condition is satisfied, the yes-pattern is used; oth- + erwise the no-pattern (if present) is used. If there are + more than two alternatives in the subpattern, a compile-time + error occurs. + + There are two kinds of condition. If the text between the + parentheses consists of a sequence of digits, the condition + is satisfied if the capturing subpattern of that number has + previously matched. The number must be greater than zero. + Consider the following pattern, which contains non- + significant white space to make it more readable (assume the + PCRE_EXTENDED option) and to divide it into three parts for + ease of discussion: + + ( \( )? [^()]+ (?(1) \) ) + + The first part matches an optional opening parenthesis, and + if that character is present, sets it as the first captured + substring. The second part matches one or more characters + that are not parentheses. The third part is a conditional + subpattern that tests whether the first set of parentheses + matched or not. If they did, that is, if subject started + with an opening parenthesis, the condition is true, and so + the yes-pattern is executed and a closing parenthesis is + required. Otherwise, since no-pattern is not present, the + subpattern matches nothing. In other words, this pattern + matches a sequence of non-parentheses, optionally enclosed + in parentheses. + + If the condition is not a sequence of digits, it must be an + assertion. This may be a positive or negative lookahead or + lookbehind assertion. Consider this pattern, again contain- + ing non-significant white space, and with the two alterna- + tives on the second line: + + (?(?=[^a-z]*[a-z]) + \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) + + The condition is a positive lookahead assertion that matches + an optional sequence of non-letters followed by a letter. In + other words, it tests for the presence of at least one + letter in the subject. If a letter is found, the subject is + matched against the first alternative; otherwise it is + matched against the second. This pattern matches strings in + one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are + letters and dd are digits. + + + +COMMENTS + The sequence (?# marks the start of a comment which contin- + ues up to the next closing parenthesis. Nested parentheses + are not permitted. The characters that make up a comment + play no part in the pattern matching at all. + + If the PCRE_EXTENDED option is set, an unescaped # character + outside a character class introduces a comment that contin- + ues up to the next newline character in the pattern. + + + +RECURSIVE PATTERNS + Consider the problem of matching a string in parentheses, + allowing for unlimited nested parentheses. Without the use + of recursion, the best that can be done is to use a pattern + that matches up to some fixed depth of nesting. It is not + possible to handle an arbitrary nesting depth. Perl 5.6 has + provided an experimental facility that allows regular + expressions to recurse (amongst other things). It does this + by interpolating Perl code in the expression at run time, + and the code can refer to the expression itself. A Perl pat- + tern to solve the parentheses problem can be created like + this: + + $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; + + The (?p{...}) item interpolates Perl code at run time, and + in this case refers recursively to the pattern in which it + appears. Obviously, PCRE cannot support the interpolation of + Perl code. Instead, the special item (?R) is provided for + the specific case of recursion. This PCRE pattern solves the + parentheses problem (assume the PCRE_EXTENDED option is set + so that white space is ignored): + + \( ( (?>[^()]+) | (?R) )* \) + + First it matches an opening parenthesis. Then it matches any + number of substrings which can either be a sequence of non- + parentheses, or a recursive match of the pattern itself + (i.e. a correctly parenthesized substring). Finally there is + a closing parenthesis. + + This particular example pattern contains nested unlimited + repeats, and so the use of a once-only subpattern for match- + ing strings of non-parentheses is important when applying + the pattern to strings that do not match. For example, when + it is applied to + + (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() + + it yields "no match" quickly. However, if a once-only sub- + pattern is not used, the match runs for a very long time + indeed because there are so many different ways the + and * + repeats can carve up the subject, and all have to be tested + before failure can be reported. + + The values set for any capturing subpatterns are those from + the outermost level of the recursion at which the subpattern + value is set. If the pattern above is matched against + + (ab(cd)ef) + + the value for the capturing parentheses is "ef", which is + the last value taken on at the top level. If additional + parentheses are added, giving + + \( ( ( (?>[^()]+) | (?R) )* ) \) + ^ ^ + ^ ^ the string they capture is + "ab(cd)ef", the contents of the top level parentheses. If + there are more than 15 capturing parentheses in a pattern, + PCRE has to obtain extra memory to store data during a + recursion, which it does by using pcre_malloc, freeing it + via pcre_free afterwards. If no memory can be obtained, it + saves data for the first 15 capturing parentheses only, as + there is no way to give an out-of-memory error from within a + recursion. + + + +PERFORMANCE + Certain items that may appear in patterns are more efficient + than others. It is more efficient to use a character class + like [aeiou] than a set of alternatives such as (a|e|i|o|u). + In general, the simplest construction that provides the + required behaviour is usually the most efficient. Jeffrey + Friedl's book contains a lot of discussion about optimizing + regular expressions for efficient performance. + + When a pattern begins with .* and the PCRE_DOTALL option is + set, the pattern is implicitly anchored by PCRE, since it + can match only at the start of a subject string. However, if + PCRE_DOTALL is not set, PCRE cannot make this optimization, + because the . metacharacter does not then match a newline, + and if the subject string contains newlines, the pattern may + match from the character immediately following one of them + instead of from the very start. For example, the pattern + + (.*) second + + matches the subject "first\nand second" (where \n stands for + a newline character) with the first captured substring being + "and". In order to do this, PCRE has to retry the match + starting after every newline in the subject. + + If you are using such a pattern with subject strings that do + not contain newlines, the best performance is obtained by + setting PCRE_DOTALL, or starting the pattern with ^.* to + indicate explicit anchoring. That saves PCRE from having to + scan along the subject looking for a newline to restart at. + + Beware of patterns that contain nested indefinite repeats. + These can take a long time to run when applied to a string + that does not match. Consider the pattern fragment + + (a+)* + + This can match "aaaa" in 33 different ways, and this number + increases very rapidly as the string gets longer. (The * + repeat can match 0, 1, 2, 3, or 4 times, and for each of + those cases other than 0, the + repeats can match different + numbers of times.) When the remainder of the pattern is such + that the entire match is going to fail, PCRE has in princi- + ple to try every possible variation, and this can take an + extremely long time. + + An optimization catches some of the more simple cases such + as + + (a+)*b + + where a literal character follows. Before embarking on the + standard matching procedure, PCRE checks that there is a "b" + later in the subject string, and if there is not, it fails + the match immediately. However, when there is no following + literal this optimization cannot be used. You can see the + difference by comparing the behaviour of + + (a+)*\d + + with the pattern above. The former gives a failure almost + instantly when applied to a whole line of "a" characters, + whereas the latter takes an appreciable time with strings + longer than about 20 characters. + + + + +AUTHOR + Philip Hazel <ph10@cam.ac.uk> + University Computing Service, + New Museums Site, + Cambridge CB2 3QG, England. + Phone: +44 1223 334714 + + Last updated: 15 August 2001 + Copyright (c) 1997-2001 University of Cambridge. |