Literals
All characters are literals except: ".", "*", "?", "+", "(", ")", "{", "}", "[", "]", "^", "$" and "\". These characters are literals when preceded by a "\". A literal is a character that matches itself.
Wildcard
The dot character "." matches any single character except ':'
Repeats
A repeat is an expression that is repeated an arbitrary number of times. An expression followed by "*" can be repeated any number of times including zero. An expression followed by "+" can be repeated any number of times, but at least once. An expression followed by "?" may be repeated zero or one times only. When it is necessary to specify the minimum and maximum number of repeats explicitly, the bounds operator "{}" may be used, thus "a{2}" is the letter "a" repeated exactly twice, "a{2,4}" represents the letter "a" repeated between 2 and 4 times, and "a{2,}" represents the letter "a" repeated at least twice with no upper limit. Note that there must be no white-space inside the {}, and there is no upper limit on the values of the lower and upper bounds. All repeat expressions refer to the shortest possible previous sub-expression: a single character; a character set, or a sub-expression grouped with "()" for example.
Examples:
"ba*" will match all of "b", "ba", "baaa" etc.
"ba+" will match "ba" or "baaaa" for example but not "b".
"ba?" will match "b" or "ba".
"ba{2,4}" will match "baa", "baaa" and "baaaa".
Non-greedy repeats
Whenever the "extended" regular expression syntax is in use (the default) then non-greedy repeats are possible by appending a '?' after the repeat; a non-greedy repeat is one which will match the shortest possible string.
For example to match html tag pairs one could use something like:
"<\s*tagname[^>]*>(.*?)<\s*/tagname\s*>"
In this case $1 will contain the text between the tag pairs, and will be the shortest possible matching string.
Parenthesis
Parentheses serve two purposes, to group items together into a sub-expression, and to mark what generated the match. For example the expression "(ab)*" would match all of the string "ababab".
Non-Marking Parenthesis
Sometimes you need to group sub-expressions with parenthesis, but don't want the parenthesis to spit out another marked sub-expression, in this case a non-marking parenthesis (?:expression) can be used. For example the following expression creates no sub-expressions:
"(?:abc)*"
Forward Lookahead Asserts
There are two forms of these; one for positive forward lookahead asserts, and one for negative lookahead asserts:
"(?=abc)" matches zero characters only if they are followed by the expression "abc".
"(?!abc)" matches zero characters only if they are not followed by the expression "abc".
Alternatives
Alternatives occur when the expression can match either one sub-expression or another, each alternative is separated by a "|". Each alternative is the largest possible previous sub-expression; this is the opposite behavior from repetition operators.
Examples:
"a(b|c)" could match "ab" or "ac".
"abc|def" could match "abc" or "def".
Sets
A set is a set of characters that can match any single character that is a member of the set. Sets are delimited by "[" and "]" and can contain literals, character ranges, character classes, collating elements and equivalence classes. Set declarations that start with "^" contain the compliment of the elements that follow.
Examples:
Character literals:
"[abc]" will match either of "a", "b", or "c".
"[^abc] will match any character other than "a", "b", or "c".
Character ranges:
"[a-z]" will match any character in the range "a" to "z".
"[^A-Z]" will match any character other than those in the range "A" to "Z".
Note that character ranges are highly locale dependent: they match any character that collates between the endpoints of the range, ranges will only behave according to ASCII rules when the default "C" locale is in effect. For example if the library is compiled with the Win32 localization model, then [a-z] will match the ASCII characters a-z, and also 'A', 'B' etc, but not 'Z' which collates just after 'z'.
Character classes are denoted using the syntax "[:classname:]" within a set declaration, for example "[[:space:]]" is the set of all whitespace characters. The available character classes are:
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alnum
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Any alpha numeric character.
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alpha
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Any alphabetical character a-z and A-Z. Other characters may also be included depending upon the locale.
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blank
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Any blank character, either a space or a tab.
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cntrl
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Any control character.
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digit
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Any digit 0-9.
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graph
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Any graphical character.
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lower
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Any lower case character a-z. Other characters may also be included depending upon the locale.
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print
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Any printable character.
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punct
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Any punctuation character.
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space
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Any whitespace character.
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upper
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Any upper case character A-Z. Other characters may also be included depending upon the locale.
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xdigit
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Any hexadecimal digit character, 0-9, a-f and A-F.
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word
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Any word character - all alphanumeric characters plus the underscore.
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unicode
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Any character whose code is greater than 255, this applies to the wide character traits classes only.
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There are some shortcuts that can be used in place of the character classes:
\w in place of [:word:]
\s in place of [:space:]
\d in place of [:digit:]
\l in place of [:lower:]
\u in place of [:upper:]
Collating elements take the general form [.tagname.] inside a set declaration, where tagname is either a single character, or a name of a collating element, for example [[.a.]] is equivalent to [a], and [[.comma.]] is equivalent to [,]. The library supports all the standard POSIX collating element names, and in addition the following digraphs: "ae", "ch", "ll", "ss", "nj", "dz", "lj", each in lower, upper and title case variations. Multi-character collating elements can result in the set matching more than one character, for example [[.ae.]] would match two characters, but note that [^[.ae.]] would only match one character.
Equivalence classes take the general form [=tagname=] inside a set declaration, where tagname is either a single character, or a name of a collating element, and matches any character that is a member of the same primary equivalence class as the collating element [.tagname.]. An equivalence class is a set of characters that collate the same, a primary equivalence class is a set of characters whose primary sort key are all the same (for example strings are typically collated by character, then by accent, and then by case; the primary sort key then relates to the character, the secondary to the accentation, and the tertiary to the case). If there is no equivalence class corresponding to tagname, then [=tagname=] is exactly the same as [.tagname.].
To include a literal "-" in a set declaration then: make it the first character after the opening "[" or "[^", the endpoint of a range, or a collating element.
Line anchors
An anchor is something that matches the null string at the start or end of a line: "^" matches the null string at the start of a line, "$" matches the null string at the end of a line.
Back references
A back reference is a reference to a previous sub-expression that has already been matched, the reference is to what the sub-expression matched, not to the expression itself. A back reference consists of the escape character "\" followed by a digit "1" to "9", "\1" refers to the first sub-expression, "\2" to the second etc. For example the expression "(.*)\1" matches any string that is repeated about its mid-point for example "abcabc" or "xyzxyz". A back reference to a sub-expression that did not participate in any match, matches the null string: NB this is different to some other regular expression matchers.
Characters by code
This is an extension to the algorithm that is not available in other libraries; it consists of the escape character followed by the digit "0" followed by the octal character code. For example "\023" represents the character whose octal code is 23. Where ambiguity could occur use parentheses to break the expression up: "\0103" represents the character whose code is 103, "(\010)3 represents the character 10 followed by "3". To match characters by their hexadecimal code, use \x followed by a string of hexadecimal digits, optionally enclosed inside {}, for example \xf0 or \x{aff}, notice the latter example is a Unicode character.
Word operators
"\w" matches any single character that is a member of the "word" character class, this is identical to the expression "[[:word:]]".
"\W" matches any single character that is not a member of the "word" character class, this is identical to the expression "[^[:word:]]".
"\<" matches the null string at the start of a word.
"\>" matches the null string at the end of the word.
"\b" matches the null string at either the start or the end of a word.
"\B" matches a null string within a word.
The start of the sequence passed to the matching algorithms is considered to be a potential start of a word.
Buffer operators
"\`" matches the start of a buffer.
"\A" matches the start of the buffer.
"\'" matches the end of a buffer.
"\z" matches the end of a buffer.
"\Z" matches the end of a buffer, or possibly one or more new line characters followed by the end of the buffer.
Escape operator
The escape character "\" has several meanings.
Inside a set declaration the escape character is a normal character.
The escape operator may introduce an operator for example: back references, or a word operator.
The escape operator may make the following character normal, for example "\*" represents a literal "*" rather than the repeat operator.
Single character escape sequences
The following escape sequences are aliases for single characters:
Escape sequence Character code Meaning
\a 0x07 Bell character.
\f 0x0C Form feed.
\n 0x0A Newline character.
\r 0x0D Carriage return.
\t 0x09 Tab character.
\v 0x0B Vertical tab.
\e 0x1B ASCII Escape character.
\0dd 0dd An octal character code, where dd is one or more octal digits.
\xXX 0xXX A hexadecimal character code, where XX is one or more hexadecimal digits.
\x{XX} 0xXX A hexadecimal character code, where XX is one or more hexadecimal digits, optionally a unicode character.
\cZ z-@ An ASCII escape sequence control-Z, where Z is any ASCII character greater than or equal to the character code for '@'.
Miscellaneous escape sequences:
\w Equivalent to [[:word:]].
\W Equivalent to [^[:word:]].
\s Equivalent to [[:space:]].
\S Equivalent to [^[:space:]].
\d Equivalent to [[:digit:]].
\D Equivalent to [^[:digit:]].
\l Equivalent to [[:lower:]].
\L Equivalent to [^[:lower:]].
\u Equivalent to [[:upper:]].
\U Equivalent to [^[:upper:]].
\C Any single character, equivalent to '.'.
\X Match any Unicode combining character sequence, for example "a\x 0301" (a letter a with an acute).
\Q The begin quote operator, everything that follows is treated as a literal character until a \E end quote operator is found.
\E The end quote operator, terminates a sequence begun with \Q.
What gets matched?
The regular expression library will match the first possible matching string.
Regular Expression documentation was modified by Blue Squirrel with permission. The following message applies to the regular expression matching library:
Permission to use, copy, modify, distribute and sell this software and its documentation for any purpose is hereby granted without fee, provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation. Dr John Maddock makes no representations about the suitability of this software for any purpose. It is provided "as is" without express or implied warranty.
Copyright Dr. John Maddock 1998-2000 all rights reserved.