Perl-compatible Regular Expression functions

Perl-compatible Regular Expression functions PCRE The syntax for patterns used in these functions closely resembles Perl. The expression should be enclosed in the delimiters, a forward slash (/), for example. Any character can be used for delimiter as long as it's not alphanumeric or backslash (\). If the delimiter character has to be used in the expression itself, it needs to be escaped by backslash. The ending delimiter may be followed by various modifiers that affect the matching. See Pattern Modifiers. Examples of valid patterns /<\/\w+>/ |(\d{3})-\d+|Sm /^(?i)php[34]/ Examples of invalid patterns /href='(.*)' - missing ending delimiter /\w+\s*\w+/J - unknown modifier 'J' 1-\d3-\d3-\d4| - missing starting delimiter preg_match Perform a regular expression match Description int preg_match string pattern string subject array matches Searches subject for a match to the regular expression given in pattern. If matches is provided, then it is filled with the results of search. $matches[0] will contain the text that match the full pattern, $matches[1] will have the text that matched the first captured parenthesized subpattern, and so on. Returns true if a match for pattern was found in the subject string, or false if not match was found or an error occurred. Getting the page number out of a string if (preg_match("/page\s+#(\d+)/i", "Go to page #9.", $parts)) print "Next page is $parts[1]"; else print "Page not found."; See also preg_match_all, preg_replace, and preg_split. preg_match_all Perform a global regular expression match Description int preg_match_all string pattern string subject array matches int order Searches subject for all matches to the regular expression given in pattern and puts them in matches in the order specified by order. After the first match is found, the subsequent searches are continued on from end of the last match. order can be one of two things: PREG_PATTERN_ORDER Orders results so that $matches[0] is an array of full pattern matches, $matches[1] is an array of strings matched by the first parenthesized subpattern, and so on. preg_match_all("|<[^>]+>(.*)</[^>]+>|U", "<b>example: </b><div align=left>a test</div>", $out, PREG_PATTERN_ORDER); print $out[0][0].", ".$out[0][1]."\n"; print $out[1][0].", ".$out[1][1]."\n" This example will produce: <b>example: </b>, <div align=left>this is a test</div> example: , this is a test So, $out[0] contains array of strings that matched full pattern, and $out[1] contains array of strings enclosed by tags. PREG_SET_ORDER Orders results so that $matches[0] is an array of first set of matches, $matches[1] is an array of second set of matches, and so on. preg_match_all("|<[^>]+>(.*)</[^>]+>|U", "<b>example: </b><div align=left>a test</div>", $out, PREG_SET_ORDER); print $out[0][0].", ".$out[0][1]."\n"; print $out[1][0].", ".$out[1][1]."\n" This example will produce: <b>example: </b>, example: <div align=left>this is a test</div>, this is a test In this case, $matches[0] is the first set of matches, and $matches[0][0] has text matched by full pattern, $matches[0][1] has text matched by first subpattern and so on. Similarly, $matches[1] is the second set of matches, etc. If order is not specified, it is assumed to be PREG_PATTERN_ORDER. Returns the number of full pattern matches, or false if no match is found or an error occurred. Getting all phone numbers out of some text. preg_match_all("/$? (\d{3})? $? (?(1) [\-\s] ) \d{3}-\d{4}/x", "Call 555-1212 or 1-800-555-1212", $phones); See also preg_match, preg_replace, and preg_split. preg_replace Perform a regular expression search and replace Description mixed preg_replace mixed pattern mixed replacement mixed subject Searches subject for matches to pattern and replaces them with replacement . replacement may contain references of the form \\n. Every such reference will be replaced by the text captured by the n'th parenthesized pattern. n can be from 0 to 99, and \\0 refers to the text matched by the whole pattern. Opening parentheses are counted from left to right (starting from 1) to obtain the number of the capturing subpattern. If no matches are found in subject, then it will be returned unchanged. Every parameter to preg_replace can be an array. If subject is an array, then the search and replace is performed on every entry of subject, and the return value is an array as well. If pattern and replacement are arrays, then preg_replace takes a value from each array and uses them to do search and replace on subject. If replacement has fewer values than pattern, then empty string is used for the rest of replacement values. If pattern is an array and replacement is a string; then this replacement string is used for every value of pattern. The converse would not make sense, though. /e modifier makes preg_replace treat the replacement parameter as PHP code after the appropriate references substitution is done. Tip: make sure that replacement constitutes a valid PHP code string, otherwise PHP will complain about a parse error at the line containing preg_replace. This modifier was added in PHP 4.0. Replacing several values $patterns = array("/(19|20\d{2})-(\d{1,2})-(\d{1,2})/", "/^\s*{(\w+)}\s*=/"); $replace = array("\\3/\\4/\\1", "$\\1 ="); print preg_replace($patterns, $replace, "{startDate} = 1999-5-27"); This example will produce: $startDate = 5/27/1999 Using /e modifier preg_replace("/(<\/?)(\w+)([^>]*>)/e", "'\\1'.strtoupper('\\2').'\\3'", $html_body); This would capitalize all HTML tags in the input text. See also preg_match, preg_match_all, and preg_split. preg_split Split string by a regular expression Description array preg_split string pattern string subject int limit Returns an array containing substrings of subject split along boundaries matched by pattern. If limit is specified, then only substrings up to limit are returned. Getting parts of search string $keywords = preg_split("/[\s,]+/", "hypertext language, programming"); See also preg_match, preg_match_all, and preg_replace. preg_quote Quote regular expression characters Description string preg_quote string str preg_quote takes str and puts a backslash in front of every character that is part of the regular expression syntax. This is useful if you have a run-time string that you need to match in some text and the string may contain special regex characters. The special regular expression characters are: . \\ + * ? [ ^ ] $ ( ) { } = ! < > | : This function was added in PHP 3.0.9. preg_grep Return array entries that match the pattern Description array preg_grep string pattern array input preg_grep returns the array consisting of the elements of the input array that match the given pattern. <function>preg_grep</function> example preg_grep("/^(\d+)?\.\d+$/", $array); // find all floating point numbers in the array This function was added in PHP 4.0. Pattern Modifiers describes possible modifiers in regex patterns Description The current possible PCRE modifiers are listed below. The names in parentheses refer to internal PCRE names for these modifiers.

i (PCRE_CASELESS) If this modifier is set, letters in the pattern match both upper and lower case letters. m (PCRE_MULTILINE) By default, PCRE treats the subject string as consisting of a single "line" of characters (even if it actually contains several newlines). The "start of line" metacharacter (^) matches only at the start of the string, while the "end of line" metacharacter ($) matches only at the end of the string, or before a terminating newline (unless E modifier is set). This is the same as Perl. When this modifier is set, the "start of line" and "end of line" constructs match immediately following or immediately before any newline in the subject string, respectively, as well as at the very start and end. This is equivalent to Perl's /m modifier. If there are no "\n" characters in a subject string, or no occurrences of ^ or $ in a pattern, setting this modifier has no effect. s (PCRE_DOTALL) If this modifier is set, a dot metacharater in the pattern matches all characters, including newlines. Without it, newlines are excluded. This modifier is equivalent to Perl's /s modifier. A negative class such as [^a] always matches a newline character, independent of the setting of this modifier. x (PCRE_EXTENDED) If this modifier is set, whitespace data characters in the pattern are totally ignored except when escaped or inside a character class, and characters between an unescaped # outside a character class and the next newline character, inclusive, are also ignored. This is equivalent to Perl's /x modifier, and makes it possible to include comments inside complicated patterns. Note, however, that this applies only to data characters. Whitespace characters may never appear within special character sequences in a pattern, for example within the sequence (?( which introduces a conditional subpattern. e If this modifier is set, preg_replace does normal substitution of \\ references in the replacement string, evaluates it as PHP code, and uses the result for replacing the search string. Only preg_replace uses this modifier; it is ignored by other PCRE functions. This modifier was added in PHP 4.0. A (PCRE_ANCHORED) If this modifier is set, the pattern is forced to be "anchored", that is, it is constrained to match only at the start of the string which is being searched (the "subject string"). This effect can also be achieved by appropriate constructs in the pattern itself, which is the only way to do it in Perl. E (PCRE_DOLLAR_ENDONLY) If this modifier is set, a dollar metacharacter in the pattern matches only at the end of the subject string. Without this modifier, a dollar also matches immediately before the final character if it is a newline (but not before any other newlines). This modifier is ignored if m modifier is set. There is no equivalent to this modifier in Perl. S When a pattern is going to be used several times, it is worth spending more time analyzing it in order to speed up the time taken for matching. If this modifier is set, then this extra analysis is performed. At present, studying a pattern is useful only for non-anchored patterns that do not have a single fixed starting character. U (PCRE_UNGREEDY) This modifier inverts the "greediness" of the quantifiers so that they are not greedy by default, but become greedy if followed by "?". It is not compatible with Perl. It can also be set by a (?U) modifier setting within the pattern. X (PCRE_EXTRA) This modifier turns on additional functionality of PCRE that is incompatible with Perl. Any backslash in a pattern that is followed by a letter that has no special meaning causes an error, thus reserving these combinations for future expansion. By default, as in Perl, a backslash followed by a letter with no special meaning is treated as a literal. There are at present no other features controlled by this modifier.

Pattern Syntax describes PCRE regex syntax Description The PCRE library is a set of functions that implement regular expression pattern matching using the same syntax and semantics as Perl 5, with just a few differences (see below). The current implementation corresponds to Perl 5.005. Differences From Perl The differences described here are with respect to Perl 5.005. 1. By default, a whitespace character is any character that the C library function isspace() recognizes, though it is possible to compile PCRE with alternative character type tables. Normally isspace() matches space, formfeed, newline, carriage return, horizontal tab, and vertical tab. Perl 5 no longer includes vertical tab in its set of whitespace char- acters. The \v escape that was in the Perl documentation for a long time was never in fact recognized. However, the char- acter itself was treated as whitespace at least up to 5.002. In 5.004 and 5.005 it does not match \s. 2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits them, but they do not mean what you might think. For example, (?!a){3} does not assert that the next three characters are not "a". It just asserts that the next character is not "a" three times. 3. Capturing subpatterns that occur inside negative looka- head assertions are counted, but their entries in the offsets vector are never set. Perl sets its numerical vari- ables from any such patterns that are matched before the assertion fails to match something (thereby succeeding), but only if the negative lookahead assertion contains just one branch. 4. Though binary zero characters are supported in the sub- ject string, they are not allowed in a pattern string because it is passed as a normal C string, terminated by zero. The escape sequence "\0" can be used in the pattern to represent a binary zero. 5. The following Perl escape sequences are not supported: \l, \u, \L, \U, \E, \Q. In fact these are implemented by Perl's general string-handling and are not part of its pat- tern matching engine. 6. The Perl \G assertion is not supported as it is not relevant to single pattern matches. 7. Fairly obviously, PCRE does not support the (?{code}) construction. 8. There are at the time of writing some oddities in Perl 5.005_02 concerned with the settings of captured strings when part of a pattern is repeated. For example, matching "aba" against the pattern /^(a(b)?)+$/ sets $2 to the value "b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves $2 unset. However, if the pattern is changed to /^(aa(b(b))?)+$/ then $2 (and $3) get set. In Perl 5.004 $2 is set in both cases, and that is also true of PCRE. If in the future Perl changes to a consistent state that is different, PCRE may change to follow. 9. Another as yet unresolved discrepancy is that in Perl 5.005_02 the pattern /^(a)?(?(1)a|b)+$/ matches the string "a", whereas in PCRE it does not. However, in both Perl and PCRE /^(a)?a/ matched against "a" leaves $1 unset. 10. PCRE provides some extensions to the Perl regular expression facilities: (a) Although lookbehind assertions must match fixed length strings, each alternative branch of a lookbehind assertion can match a different length of string. Perl 5.005 requires them all to have the same length. (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ meta- character matches only at the very end of the string. (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special meaning is faulted. (d) If PCRE_UNGREEDY is set, the greediness of the repeti- tion quantifiers is inverted, that is, by default they are not greedy, but if followed by a question mark they are. 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 1-56592-257-3), covers them in great detail. The description here is intended as reference documentation. 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. 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. 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. 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 is it 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 character, but not (by default) newline. If the PCRE_DOTALL option is set, then dots match newlines as well. The han- dling of dot is entirely independent of the handling of cir- cumflex 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. 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. 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 pattern 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 due to the greediness of the .* item. However, if a quantifier is followed by a question mark, then 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) then 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, then 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, then 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, then 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, then 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", "ababaa" 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". Furthermore, 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". Assertion subpatterns are not capturing subpatterns, and may not be repeated, because it makes no sense to assert the same thing several times. If an assertion contains capturing subpatterns within it, these are always counted for the pur- poses of numbering the capturing subpatterns in the whole pattern. Substring capturing is carried out for positive assertions, but it does not make sense for negative asser- tions. 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 it. 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$ then the initial .* matches the entire string at first, but when this fails, it backtracks 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 pat- tern is written as ^(?>.*)(?<=abcd) then 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 process- ing time. 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, then the con- dition is satisfied if the capturing subpattern of that number has previously matched. Consider the following pat- tern, 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 continues 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. 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.