Regular Expressions

Background

Borland BCB4 and BCB5 ship with a PERL-compatible regular expression library which appears to be adapted from the open source PCRE (PERL-Compatible Regular Expressions) project.  The most current version of PCRE is available by FTP from ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/.

You can find more information about using PCRE in the Borland help by searching for "Perl-compatible regular expressions."  The source code shipped with BCB is in the [bcbroot]\Source\RTL\pcre folder.  The source is, however, not the complete PCRE distribution so, if you are interested in the documentation, sample and test programs, etc., you will need to get the full distribution from the FTP site above.

Note:  In the help file, all of the backslashes are doubled.  This would be correct if you are putting the regular expressions in a string in your program (the compiler converts "\\" in a string to a single backslash).  However, they should not be doubled when being supplied as input in, say, an edit box.  For example, use "\\d" (matches a single  decimal digit) in a string in your program; a user would key "\d" into an edit box.  I have removed the backslash doubling from the information below since it is visually distracting and potentially misleading.

Regular expression details

The syntax and semantics of the regular expressions supported 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 characters 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 pattern.  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 a non-alphanumeric character follows it, 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 following character would otherwise be interpreted as a meta-character, so it is always safe to precede a non-alphanumeric with "\" to specify that it stands for itself.  In particular, if you want to match a backslash, you write "\\".

If a pattern is compiled with the PCRE_EXTENDED option, whitespace 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 characters, 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 back reference

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 rereads up to three octal digits following 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 introduced 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.
The third use of backslash is for specifying generic character 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 controlled by PCRE's character tables, and may vary if locale- specific matching is taking place.  For example, in the "fr" (French) locale, some character 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 assertions.  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.

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 alternatives 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 constructs that can cause a pattern to be anchored.)

A dollar character is an assertion that 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.  Consequently, patterns that are anchored in single line mode because all branches start with "^" are not anchored in multiline 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.

Note:  The TRichEdit component implements the Windows Rich Edit 1.0 control which delimits paragraphs with "\r\n" (CRLF).  Therefore, to match the end of line (paragraph), you must precede "$" with "\r" or some other pattern that will match a carriage return prior to the newline.  Worse yet, you should expect rather odd behavior whenever using "\r" or "\n" -- try matching either one of these alone as the search pattern and search repeatedly.  Also, Rich Edit 2.0+ uses "\r" (CR) to delimit lines (paragraphs) and, therefore, should fail miserably with the unmodified PCRE library.

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 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, terminated 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 circumflex, 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 circumflex, 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 convenient 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 caseful 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 interpreted as indicating a range, typically as the first or last character in the class.  It is not possible to have the character "]" as the end character of a range, since a sequence such as [w-] is interpreted as a class of two characters.  The octal or hexadecimal representation of "]" can, however, 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 restricted set of characters than the matching lower case type.  For example, the class [^\W_] matches any letter or digit, but not underscore.

All non-alphanumeric 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 alternatives 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" settings apply to the whole pattern (unless there are other changes inside subpatterns).  If there is more than one setting 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 behavior 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 behavior otherwise.

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 subpattern does two things:

1.	It localizes a set of alternatives.  For example, the pattern

cat(aract|erpillar|)

matches one of the words "cat", "cataract", or "caterpillar".  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 portion 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 capturing 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 fulfill two functions is not always helpful.  There are often times when a grouping subpattern 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 substrings 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 . (period) meta-character
•	a character class
•	a back reference
•	a parenthesized subpattern (unless it is an assertion)

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 quantifier, 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 permitted 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 comments by applying the pattern

/\*.*\*/

to the string

/* first comment */  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 preferred number of matches.  Do not confuse this use of question 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 behavior.

When a parenthesized subpattern is quantified with a minimum repeat count that is greater than 1 or with a limited maximum, more store is required for the compiled pattern, in proportion to the size of the minimum or maximum.

If a pattern starts with .* then it is implicitly anchored, since whatever follows will be tried against every character position in the subject string.  PCRE treats this as though it were preceded by \A.

When a capturing subpattern is repeated, the value captured is the substring that matched the final iteration.  For example, after

(tweedle[dume]{3}\s*)+

has matched "tweedledum tweedledee" the value of the captured substring is "tweedledee".  However, if there are nested capturing subpatterns, the corresponding captured values may have been set in previous iterations.  For example, 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.

A back reference matches whatever actually matched the capturing 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 responsibility", 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 caselessly.

There may be more than one back reference to the same subpattern.  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 pattern

(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 iteration.  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 complicated 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 assertions 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 different 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 branch can match two different lengths, but it is acceptable if rewritten to use two 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.

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 purposes 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 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 reevaluated 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 reevaluated 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.  Backtracking past it to previous items, however, works as normal.

An alternative description is that a subpattern of this type matches the string of characters that an identical standalone 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 though of as a maximizing 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 complicated subpatterns, and it can be nested.

Conditional subpatterns

It is possible to cause the matching process to obey a subpattern 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; otherwise 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 condition is satisfied if the capturing subpattern of that number has previously matched.  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 containing non-significant white space, and with the two alternatives 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 continues 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 behavior is usually the most efficient.