DataFlux Data Management Studio 2.5: User Guide

Customize - Regex Library Editor - Using Regular Expressions

DataFlux Data Management Studio supports Perl-compatible syntax for regular expressions. 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. For example, the pattern: 

The quick brown fox

matches a portion of a subject string identical to itself. The power of regular expressions lies in their 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.

A substitution is a string inserted into the output of a regular expression call in place of the pattern matched by the regular expression. To further the example, suppose the following regular expression and substitution are applied to the input string:

Result:  The quick blue fox

In a substitution string, all characters stand for themselves, except the backslash-escape digit is interpreted as a back reference to a captured subpattern. See information on capturing subpatterns with regular expressions. Also, casing operators \U, \u, \L, \l, and \E may be used in substitution strings just like Perl substitutions. For additional information on Perl operators, refer to the Perl documentation.

Meta-Characters

There are two different sets of meta-characters: those recognized anywhere in the pattern except within square brackets ([ ]), and those recognized in square brackets. Outside square brackets, the meta-characters are:

The part of a pattern in square brackets is called a character class. In a character class, the only meta-characters are:

The following sections describe the use of each of the meta-characters.

Backslash

The backslash character (\) has several uses. First, if it is followed by a non-alphanumeric character, it takes away any special meaning the character has. 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 character with \ to specify that it stands for itself. For example, if you want to match a backslash, type \\.

If you want to remove the special meaning from a sequence of characters, you can do so by putting them between \Q and \E. This is different from Perl in that $ and @ are handled as literals in \Q...\E sequences in DataFlux, whereas in Perl, $ and @ cause variable interpolation. Note the following examples:

The \Q...\E sequence is recognized both inside and outside character classes.

Non-printing Characters

Second, the 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. However, when you are preparing a pattern by text editing, it is usually easier to use one of the following escape sequences than the binary character it represents:

The precise effect of \cx is as follows: if x is a lowercase character, it is converted to uppercase. 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. These digits can be in upper or lowercase. Any number of hexadecimal digits may appear between \x{ and }, but the value of the character code must be less than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters other than hexadecimal digits appear between \x{ and }, or if there is no terminating }, this form of escape is not recognized. Instead, the initial \x will be interpreted as a basic hexadecimal escape, with no following digits, giving a character whose value is zero.

Characters whose value is less than 256 can be defined by either of the two syntaxes for \x. There is no difference in the way they are handled. For example, \xdc is exactly the same as \x{dc}.

After \0 up to two further octal digits are read. 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 (code value 7). Make sure you supply two digits after the initial zero if the pattern 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, DataFlux 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. See a description of how this works under Back Reference.

Inside a character class, or if the decimal number is greater than 9 and there have not been many capturing subpatterns, Blue Fusion re-reads up to three octal digits following the backslash, and then generates a single byte from the least significant 8 bits of the value. Any subsequent digits stand for themselves. For example:

Note that you must not introduce octal values of 100 or greater because no more than three octal digits are ever read.

You can use all the sequences that define a single byte value both inside and outside character classes. In addition, inside a character class, the sequence \b is interpreted as the backspace character (hex 08). The sequence \x is interpreted as the character X. Outside a character class, these sequences have different meanings, see Generic Character Types.

Generic Character Types

The third use of backslash is to specify generic character types:

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.

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.

For compatibility with Perl, \s does not match the VT character (code 11). This makes it different from the POSIX space class. The \s characters are HT (9), LF (10), FF (12), CR (13), and space (32). (If "use locale;" is included in a Perl script, \s may match the VT character. In DataFlux, it never does.)

A "word" character is an underscore or any character that is a letter or digit. The definition of letters and digits is controlled by the DataFlux Unicode character tables.

Unicode Character Properties

Three additional escape sequences to match Unicode character properties are available. They are:

The property names represented by xx above are limited to the Unicode script names, the general category properties, and "Any", which matches any character (including newline). Other properties such as "InMusicalSymbols" are not currently supported by DataFlux. Note that \P{Any} does not match any characters, so always causes a match failure.

Sets of Unicode characters are defined as belonging to certain scripts. A character from one of these sets can be matched using a script name. For example:

\p{Greek}
\P{Han}

Those that are not part of an identified script are lumped together as "Common". The current list of scripts includes:

Arabic, Armenian, Bengali, Bopomofo, Braille, Buginese, Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic, Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin, Limbu, Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, and Yi

Each character has exactly one general category property, specified by a two-letter abbreviation. For compatibility with Perl, negation can be specified by including a circumflex between the opening brace and the property name. For example, \p{^Lu} is the same as \P{Lu}.

If only one letter is specified with \p or \P, it includes all the general category properties starting with that letter. In this case, in the absence of negation, the curly brackets in the escape sequence are optional; these two examples have the same effect:

\p{L}
\pL

These general category property codes are supported:

The special property L& is also supported; it matches a character that has the Lu, Ll, or Lt property (a letter that is not classified as a modifier or "other").

The long synonyms for these properties that Perl supports (such as, \p{Letter}) are not supported by DataFlux, and it is not permitted to prefix any of these properties with Is.

No character that is in the Unicode table has the Cn (unassigned) property. Instead, this property is assumed for any code point that is not in the Unicode table.

Specifying caseless matching does not affect these escape sequences. For example, \p{Lu} always matches only uppercase letters.

The \X escape matches any number of Unicode characters that form an extended Unicode sequence. The \X is equivalent to:

(?>\PM\pM*)

That is, it matches a character without the "mark" property, followed by zero or more characters with the "mark" property, and treats the sequence as an atomic group. Characters with the "mark" property are typically accents that affect the preceding character.

Matching characters by Unicode property is not fast because DataFlux must search a structure that contains data for over fifteen thousand characters.

Simple Assertions

The fourth use for backslash is certain simple assertions. An assertion specifies a condition that must 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 include:

You cannot use these assertions in character classes, but note that \b has a different meaning, 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 (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 in that they only match at the beginning and end of the subject string. The difference between \Z and \z is that \Z matches before a newline at the end of the string as well as at the very end, 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 that is true only if the current matching point is at the beginning 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 character 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 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.

Full Stop (Period, Dot)

Outside a character class, a dot (.) in the pattern matches any one character in the subject string except (by default) a character that signifies the end of a line. The matched character may be more than one byte long. When a line ending is defined as a single character (CR or LF), dot never matches that character; when the two-character sequence CRLF is used, dot does not match CR if it is immediately followed by LF, but otherwise it matches all characters (including isolated CRs and LFs).

The handling of dot is entirely independent of the handling of circumflex and dollar. The only relationship is that they all involve newline characters. Dot has no special meaning in a character class.

Matching a Single Byte

Outside a character class, the escape sequence \C matches any one byte. Unlike a dot, it always matches CR and LF. The feature is provided in Perl in order to match individual bytes in UTF-8 mode. Since it breaks up UTF-8 characters into individual bytes, what remains in the string may be a malformed UTF-8 string. For this reason, the \C escape sequence is best avoided.

DataFlux does not allow \C to appear in look behind assertions, because this would make it impossible to calculate the length of the look behind for UTF-8 characters.

Square Brackets and Character Classes

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 may occupy more than one byte. A matched character must be in the set of characters defined by the class, unless the first character in the class definition 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 lowercase vowel. Note that a circumflex is just a convenient notation for specifying the characters that 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.

Characters with values greater than 255 can be included in a class as a literal string of bytes, or by using the \x{ escaping mechanism.

When caseless matching is set, any letters in a class represent both their upper and lowercase 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.

Characters that might indicate line breaks (CR and LF) are never treated in any special way when matching character classes.

You can use the minus/hyphen (-) character 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 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 -) followed 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 interpreted as a single class containing a range followed by two separate characters. You can also use the octal or hexadecimal representation of ] to end a range.

Ranges operate in the collating sequence of character values. They can also be used for characters specified numerically, for example [\000-\037]. Ranges can include characters whose values are greater than 255, for example [\x{100}-\x{2ff}]. 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.

The character types \d, \D, \p, \P, \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 uppercase character types to specify a more restricted set of characters than the matching lowercase type. For example, the class [^\W_] matches any letter or digit, but not underscore.

The only meta-characters that are recognized in character classes are backslash, hyphen (only where it can be interpreted as specifying a range), circumflex (only at the start), opening square bracket (only when it can be interpreted as introducing a POSIX class name, see POSIX Character Classes), and the terminating closing square bracket. However, escaping other non-alphanumeric characters does no harm.

POSIX Character Classes

Perl supports the POSIX notation for character classes. This uses names enclosed by [: and :] within the enclosing square brackets. DataFlux also supports this notation. For example,

[01[:alpha:]%}

matches "0", "1", any alphabetic character, or %. The supported class names include:

The space characters are HT (9), LF (10), VT (11), FF (12), CR (13), and space (32). Notice that this list includes the VT character (code 11). This makes space different from \s, which does not include VT (for Perl compatibility).

The name word is a Perl extension, and blank is a GNU extension from Perl version 5.8. Another Perl extension is negation, which is indicated by a ^ character after the colon. For example,

[12[:^digit:]]

matches "1", "2", or any non-digit. DataFlux (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

You can use the vertical bar character (|) 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 alternative 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 Settings

Some matching options can be changed from within the pattern by a sequence of Perl option letters enclosed between (? and ). The option letters are:

For example, (?im) sets caseless, multi-line matching.

When an option change occurs at top level (that is, not inside subpattern parentheses), the change applies to the remainder of the pattern that follows. If the change is placed right at the start of a pattern, DataFlux extracts it into the global options for that expression.

An option change within a subpattern affects only that part of the current pattern that follows, so

(a(?i)b)c

matches abc and aBc and no other strings. 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 strange behavior otherwise).

Subpatterns

Subpatterns are delimited by parentheses ( and ), which you can nest. Marking part of a pattern as a subpattern does two things:

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.

It also sets up the subpattern as a capturing subpattern. When the whole pattern matches, that portion of the subject string that matched the subpattern is passed back to the caller through the ovector argument pcre_exec(). Opening parentheses are counted from left to right (starting from 1) to obtain numbers for 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, respectively.

The fact that 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 a question mark and a colon (?:, 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 capturing subpatterns is 65535, and the maximum depth of nesting 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 literal data character
• the . meta-character
• the \C escape sequence
• the \X escape sequence
• an escape such as \d that matches a single character
• a character class
• a back reference (see Atomic Grouping and Possessive Quantifiers)
• 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 braces ({ and }), 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 three successive vowels, but may match many more, while:

\d{8}

matches exactly eight digits. An opening brace 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.

Quantifiers apply to UTF-8 characters rather than to individual bytes. For example, \x{100}{2} matches two UTF-8 characters, each of which is represented by a two-byte sequence. Similarly, when Unicode property support is available, \X{3} matches three Unicode extended sequences, each of which may be several bytes long (and they may be of different lengths).

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 DataFlux used to give an error 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 does not match any characters, the loop is forcibly broken.

By default, the quantifiers 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 causes problems is in trying to match comments in C programs. These appear between /* and */ and within the comment, 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 because of the .* item.

However, if a quantifier is followed by a question mark, it does not match the maximum number, 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.

These quantifiers try to match the maximum number by default, but individual ones can be made to match a minimum by following them with a question mark. In other words, it inverts the default behavior.

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".

Atomic Grouping and Possessive Quantifiers

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 to fail earlier, when the author of the pattern knows there is no point in carrying on.

For example, the pattern \d+one when applied to the subject line:

123456two

After matching all 6 digits and then failing to match "one", the normal action of the match is to try again with only 5 digits matching the \d+ item, and then 4, and so on, before failing. "Atomic grouping" (a term used in Mastering Regular Expressions by Jeffrey Friedl) provides the means for specifying that once a subpattern has matched, it is not to be re-evaluated in this way.

If we use atomic grouping for the previous example, the matcher gives up immediately on failing to match "one" the first time. The notation is a kind of special parenthesis, starting with (?> as in this example:

(?>\d+)one

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 matches, if anchored at the current point in the subject string.

Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as the above example can be thought 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.

Atomic groups in general can contain arbitrarily complicated subpatterns, and can be nested. However, when the subpattern for an atomic group is just a single repeated item, as in the example above, a simpler notation, called a "possessive quantifier" can be used. This consists of an additional + character following a quantifier. Using this notation, the previous example can be rewritten as:

\d++one

Possessive quantifiers are always greedy. They are a convenient notation for the simpler forms of atomic group. However, there is no difference in the meaning or processing of a possessive quantifier and the equivalent atomic group. The possessive quantifier syntax is an extension to the Perl syntax.

When a pattern contains an unlimited repeat inside a subpattern that can itself be repeated an unlimited number of times, the use of an atomic group is the only way to avoid some failing matches taking a very long time. The pattern:

(\D+|<\d+>)*[!?]

matches an unlimited number of substrings that either consist 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 internal \D+ repeat and the external * repeat in a large number of ways, and all have to be tried. (The example uses [!?] rather than a single character at the end, because both DataFlux 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 so that it uses an atomic group, like this:

((?>\D+)|<\d+>)*[!?]

sequences of non-digits cannot be broken, and failure happens quickly.

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 (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 "forward back reference" of this type can make sense when a repetition is involved and the subpattern to the right has participated in an earlier iteration. See Backslash for more information on the handling of digits following a backslash.

A back reference matches whatever actually matched the capturing subpattern in the current subject string, rather than anything matching the subpattern itself (see Subpatterns as Subroutines). 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, 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, 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 many capturing parentheses in a pattern, 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. Here an empty comment (see Comments) 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, ababbaa, and so on. At each iteration of the subpattern, the back reference matches the character string corresponding to the previous iteration. 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 0.

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 it does not cause the current matching position to be changed. An assertion subpattern is matched in the normal way, except that it does not cause the current matching position to be changed.

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.

Look Ahead Assertions

Look ahead 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:

one(?!two)

matches any occurrence of "one" that is not followed by "two." Note that the apparently similar pattern:

(?!one)two

does not find an occurrence of "two" that is preceded by something other than "one"; it finds any occurrence of "two", because the assertion (?!one) is always true when the next three characters are "two." A look behind assertion is needed to achieve this effect.

If you want to force a matching failure at some point in a pattern, the most convenient way to do it is with (?!) because an empty string always matches, so an assertion that requires there not to be an empty string must always fail.

Look Behind Assertions

Look behind assertions start with (?<= for positive assertions and (?<! negative assertions. For example:

(?<!one)two

does find an occurrence of "two" that is not preceded by "one." The contents of a look behind assertion are restricted so 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. For example:

(?<=leopard|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 look behind assertion. This is an extension compared with Perl (for version 5.8), 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 rewritten to use two top-level branches:

(?<=abc|abde)

The implementation of look behind 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.

DataFlux does not allow the \C escape to appear in look behind assertions, because it makes it impossible to calculate the length of the look behind for UTF-8 characters. The \X escape, which can match different numbers of bytes, is also not permitted.

Atomic groups 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 that does not match. Because matching proceeds from left to right, DataFlux looks for each a in the subject and then sees 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 backtracks to match all but the last character, and 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)

or, equivalently, using the possessive quantifier syntax,

^.*+(?<=abcd)

there can be no backtracking for the .* item; it can match only the entire string. The subsequent look behind 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.

Using Multiple Assertions

Several assertions (of any sort) may occur in succession. For example:

(?<=\d{3})(?<!999)one

matches "one" 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 "one" preceded by six characters, the first of which are digits and the last three of which are not 999. For example, it does not match 123abcone. A pattern to do that is:

(?<=\d{3}...)(?<!999)one

This time, the first assertion looks at the preceding six characters, checking that the first three are digits, and then the second assertion checks the preceding three characters are not 999.

Assertions can be nested in any combination. For example:

(?<=(?<!one)two)cat

matches an occurrence of "cat" that is preceded by "two," which in turn is not preceded by "one," while:

(?<=\d{3}(?!999)...)one

is another pattern which matches "one" preceded by three digits and any three characters that are not 999.

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 three kinds of conditions. If the text between the parentheses consists of a sequence of digits, or a sequence of alphanumeric characters and underscores, the condition is satisfied, if the capturing subpattern of that number or name previously matched. There is a possible ambiguity here, because subpattern names may consist entirely of digits. DataFlux first looks for a named subpattern; if it cannot find one and the text consists entirely of digits, it looks for a subpattern of that number, which must be greater than zero. Using subpattern names that consist entirely of digits is not recommended.

Consider the following pattern, which contains non-significant white space (to make it more readable 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, if the subject started with an opening parenthesis, the condition is true, and 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. Rewriting it to use a named subpattern gives this:

(?P \( )? [^()]+ (?(OPEN) \) )

If the condition is the string (R), and there is no subpattern with the name R, the condition is satisfied if a recursive call to the pattern or subpattern has been made. At "top level", the condition is false. This is a DataFlux extension. Recursive patterns are described in the next section.

If the condition is not a sequence of digits or (R), it must be an assertion. This may be a positive or negative look ahead or look behind 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 look ahead 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 that 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.

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 provides a facility that allows regular expressions to recurse (among 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 pattern 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. DataFlux cannot support the interpolation of Perl code. Instead, it supports special syntax for recursion of the entire pattern, and for individual subpattern recursion.

The special item that consists of (? followed by a number greater than zero and a closing parenthesis is a recursive call of the subpattern of the given number, provided that it occurs inside that subpattern. (If not, it is a "subroutine" call, which is described in Subpatterns as Subroutines). The special item (?R) is a recursive call of the entire regular expression.

A recursive subpattern call is always treated as an atomic group. That is, once it has matched some of the subject string, it is never re-entered, even if it contains untried alternatives and there is a subsequent matching failure.

This pattern solves the nested parentheses problem:

\( ( (?>[^()]+) | (?R) )* \)

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 (that is, a correctly parenthesized substring). Finally there is a closing parenthesis.

If this were part of a larger pattern, you would not want to recurse the entire pattern, so instead you could use this:

( \( ( (?>[^()]+) | (?1) )* \) )

The pattern is in parentheses, and causes the recursion to refer to them instead of the whole pattern. In a larger pattern, keeping track of parenthesis numbers can be tricky. It may be more convenient to use named parentheses instead. For this, DataFlux uses (?P>name), which is an extension to the Python syntax that DataFlux uses for named parentheses (Perl does not provide named parentheses). You could rewrite the above example as follows:

(?P \( ( (?>[^()]+) | (?P>pn) )* \) )

This particular example pattern contains nested unlimited repeats, and so the use of atomic grouping for matching strings of non-parentheses is important when applying the pattern to strings that do not match. For example, when this pattern is applied to:

(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

it yields "no match" quickly. However, if atomic grouping is not used, the match runs for a very long time because there are so many different ways the + and * repeats can separate the subject, and all have to be tested before failure can be reported.

At the end of a match, the values set for any capturing subpatterns are those from the outermost level of the recursion at which the subpattern value is set. If you want to obtain intermediate values, a callout function can be used (see Subpatterns as Subroutines and the precallout documentation). 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, DataFlux has to obtain extra memory to store data during a recursion. If no memory can be obtained, the match fails with an error.

Do not confuse the (?R) item with the condition (R), which tests for recursion. Consider this pattern, which matches text in angle brackets, allowing for arbitrary nesting. Only digits are allowed in nested brackets during recursion, whereas any characters are permitted at the outer level.

< (?: (?(R) \d++ | [^<>]*+) | (?R)) * >

In this pattern, (?(R) is the start of a conditional subpattern, with two different alternatives for the recursive and non-recursive cases. The (?R) item is the actual recursive call.

Subpatterns as Subroutines

If the syntax for a recursive subpattern reference (either by number or by name) is used outside the parentheses to which it refers, it operates like a subroutine in a programming language. An earlier example demonstrated that the pattern:

(sens|respons)e and \1ibility

matches "sense and sensibility" and "response and responsibility", but not "sense and responsibility". If the pattern:

(sens|respons)e and (?1)ibility

is used, it matches "sense and responsibility" as well as the other two strings. Such references, if given numerically, must follow the subpattern to which they refer. However, named references can refer to later subpatterns.

Like recursive subpatterns, a "subroutine" call is always treated as an atomic group. That is, once it has matched some of the subject string, it is never re-entered, even if it contains untried alternatives and there is a subsequent matching failure.

Unicode Support

Special information regarding support of Unicode characters is given below:

1. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte UTF-8 character if the value is greater than 127.
2. Octal numbers up to \777 are recognized, and match two-byte UTF-8 characters for values greater than \177.
3. Repeat quantifiers apply to complete UTF-8 characters, not to individual bytes, for example: \x{100}{3}.
4. The dot (.) meta-character matches one UTF-8 character instead of a single byte.
5. The escape sequence \C can be used to match a single byte in UTF-8 mode, but its use can lead to some strange results.
6. Similarly, characters that match the POSIX named character classes are all low-valued characters.

Differences with Perl

This section describes the differences in the ways that DataFlux and Perl handle regular expressions. The differences described here compare DataFlux with Perl version 5.8.

1. DataFlux has only a subset of the Perl UTF-8 and Unicode support. Details of what it does have are given above.
2. DataFlux does not allow repeat quantifiers on look ahead 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 asserts that the next character is not "a" three times.
3. Capturing subpatterns that occur inside negative look ahead assertions are counted, but their entries in the offsets vector are never set. Perl sets its numerical variables from any patterns that are matched before the assertion fails to match something (thereby succeeding), but only if the negative look ahead assertion contains just one branch.
4. Though binary zero characters are supported in the subject 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 properties that can be tested with \p and \P are limited to the general category properties such as Lu and Nd, script names such as Greek or Han, and the derived properties Any and L&.
6. DataFlux does support the \Q...\E escape for quoting substrings. Characters in between are treated as literals. This is slightly different from Perl in that $ and @ are also handled as literals inside the quotes. In Perl, they cause variable interpolation (but of course DataFlux does not have variables). Note these examples:
   
   Note: The \Q...\E sequence is recognized both inside and outside character classes.

Pattern	DataFlux Matches	Perl Matches
\Qabc$xyz\E	abc$xyz	abc followed by the contents of $xyz
\Qabc\$xyz\E	abc\$xyz	abc\$xyz
\Qabc\E\$\Qxyz\E	abc$xyz	abc$xyz

7. DataFlux does not support the (?{code}) and (?p{code}) constructions. However, there is support for recursive patterns using the non-Perl items (?R), (?number), and (?P>name).
8. There are some differences that are concerned with the settings of captured strings when part of a pattern is repeated. For example, matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in DataFlux it is set to "b".
9. DataFlux provides some extensions to the Perl regular expression facilities:

1. Although look behind assertions must match fixed length strings, each alternative branch of a look behind assertion can match a different length of string. Perl requires them all to have the same length.
2. Where a backslash is followed by a letter with no special meaning, the backslash is always ignored (Perl can be made to issue a warning).
3. The greediness of the repetition quantifiers is inverted; that is, by default they are not greedy, but if followed by a question mark they are.
4. The (?R), (?number), and (?P>name) constructs allows for recursive pattern matching (Perl can do this using the (?p{code}) construct, which PCRE cannot support.)
5. DataFlux supports the possessive quantifier "++" syntax, taken from the Sun© Java© package.

Related Topics

• Customize - Regex Library Editor
• Customize - Regex Library Editor - Building Regex Libraries