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Regular expressions



 This section is part of the wxWidgets documentation for "Syntax of the builtin regular expression library". The parts which do not apply to Merge have been omitted.


  • RE : Regular Expression
  • ARE : Advanced Regular Expression
  • A regular expression describes strings of characters. It's a pattern that matches certain strings and doesn't match others.

Regular Expression Syntax
Bracket Expressions
Regular Expression Character Names


Regular Expression Syntax

These regular expressions are implemented using the package written by Henry Spencer, based on the 1003.2 spec and some (not quite all) of the Perl5 extensions (thanks, Henry!). Much of the description of regular expressions below is copied verbatim from his manual entry.

An ARE is one or more branches, separated by '|', matching anything that matches any of the branches.

A branch is zero or more constraints or quantified atoms, concatenated. It matches a match for the first, followed by a match for the second, etc; an empty branch matches the empty string.

A quantified atom is an atom possibly followed by a single quantifier. Without a quantifier, it matches a match for the atom. The quantifiers, and what a so-quantified atom matches, are:

* a sequence of 0 or more matches of the atom
+ a sequence of 1 or more matches of the atom
? a sequence of 0 or 1 matches of the atom
{m} a sequence of exactly m matches of the atom
{m,} a sequence of m or more matches of the atom
{m,n} a sequence of m through n (inclusive) matches of the atom; m may not exceed n
*? +? ?? {m}? {m,}? {m,n}? non-greedy quantifiers, which match the same possibilities, but prefer the smallest number rather than the largest number of matches

The forms using { and } are known as bounds. The numbers m and n are unsigned decimal integers with permissible values from 0 to 255 inclusive. An atom is one of:

(re) (where re is any regular expression) matches a match for re, with the match noted for possible reporting
(?:re) as previous, but does no reporting (a "non-capturing'' set of parentheses)
() matches an empty string, noted for possible reporting
(?:) matches an empty string, without reporting
[chars] a bracket expression, matching any one of the chars (see Bracket Expressions for more detail)
. matches any single character
\k (where k is a non-alphanumeric character) matches that character taken as an ordinary character, e.g. \\ matches a backslash character
\c where c is alphanumeric (possibly followed by other characters), an escape , see Escapes below
{ when followed by a character other than a digit, matches the left-brace character '{'; when followed by a digit, it is the beginning of a bound (see above)
x where x is a single character with no other significance, matches that character.

A constraint matches an empty string when specific conditions are met. A constraint may not be followed by a quantifier. The simple constraints are as follows; some more constraints are described later, under Escapes.

^ matches at the beginning of a line
$ matches at the end of a line
(?=re) positive lookahead , matches at any point where a substring matching re begins
(?!re) negative lookahead , matches at any point where no substring matching re begins

The lookahead constraints may not contain back references (see later), and all parentheses within them are considered non-capturing.

An RE may not end with '\'.


Bracket Expressions

A bracket expression is a list of characters enclosed in '[]'. It normally matches any single character from the list (but see below). If the list begins with '^', it matches any single character (but see below) not from the rest of the list.

If two characters in the list are separated by '-', this is shorthand for the full range of characters between those two (inclusive) in the collating sequence, e.g. [0-9] in ASCII matches any decimal digit. Two ranges may not share an endpoint, so e.g. a-c-e is illegal. Ranges are very collating-sequence-dependent, and portable programs should avoid relying on them.

To include a literal ] or - in the list, the simplest method is to enclose it in [. and .] to make it a collating element (see below). Alternatively, make it the first character (following a possible '^'), or precede it with '\'. Alternatively, for '-', make it the last character, or the second endpoint of a range. To use a literal - as the first endpoint of a range, make it a collating element or (AREs only) precede it with '\'. With the exception of these, some combinations using [ (see next paragraphs), and escapes, all other special characters lose their special significance within a bracket expression.

Within a bracket expression, a collating element (a character, a multi-character sequence that collates as if it were a single character, or a collating-sequence name for either) enclosed in [. and .] stands for the sequence of characters of that collating element.

Within a bracket expression, a collating element enclosed in [= and =] is an equivalence class, standing for the sequences of characters of all collating elements equivalent to that one, including itself. An equivalence class may not be an endpoint of a range.

Within a bracket expression, the name of a character class enclosed in [: and :] stands for the list of all characters (not all collating elements!) belonging to that class. Standard character classes are:

alpha A letter.
upper An upper-case letter.
lower A lower-case letter.
digit A decimal digit.
xdigit A hexadecimal digit.
alnum An alphanumeric (letter or digit).
print An alphanumeric (same as alnum).
blank A space or tab character.
space A character producing white space in displayed text.
punct A punctuation character.
graph A character with a visible representation.
cntrl A control character.

A character class may not be used as an endpoint of a range.

There are two special cases of bracket expressions: the bracket expressions [[:<:]] and [[:>:]] are constraints, matching empty strings at the beginning and end of a word respectively. A word is defined as a sequence of word characters that is neither preceded nor followed by word characters. A word character is an alnum character or an underscore (_). These special bracket expressions are deprecated; users of AREs should use constraint escapes instead (see Escapes below).



Escapes , which begin with a \ followed by an alphanumeric character, come in several varieties: character entry, class shorthands, constraint escapes, and back references. A \ followed by an alphanumeric character but not constituting a valid escape is illegal in AREs.

Character-entry escapes exist to make it easier to specify non-printing and otherwise inconvenient characters in REs:

\a alert (bell) character, as in C
\b backspace, as in C
\B synonym for \ to help reduce backslash doubling in some applications where there are multiple levels of backslash processing
\cX (where X is any character) the character whose low-order 5 bits are the same as those of X, and whose other bits are all zero
\e the character whose collating-sequence name is 'ESC', or failing that, the character with octal value 033
\f formfeed, as in C
\n newline, as in C
\r carriage return, as in C
\t horizontal tab, as in C
\uwxyz (where wxyz is exactly four hexadecimal digits) the Unicode character U+wxyz in the local byte ordering
\Ustuvwxyz (where stuvwxyz is exactly eight hexadecimal digits) reserved for a somewhat-hypothetical Unicode extension to 32 bits
\v vertical tab, as in C are all available.
\xhhh (where hhh is any sequence of hexadecimal digits) the character whose hexadecimal value is 0xhhh (a single character no matter how many hexadecimal digits are used).
\0 the character whose value is 0
\xy (where xy is exactly two octal digits, and is not a back reference (see below)) the character whose octal value is 0xy
\xyz (where xyz is exactly three octal digits, and is not a back reference (see below)) the character whose octal value is 0xyz

Hexadecimal digits are '0'-'9', 'a'-'f', and 'A'-'F'. Octal digits are '0'-'7'.

The character-entry escapes are always taken as ordinary characters. For example, \135 is ] in ASCII, but \135 does not terminate a bracket expression. Beware, however, that some applications (e.g., C compilers) interpret such sequences themselves before the regular-expression package gets to see them, which may require doubling (quadrupling, etc.) the '\'.

Class-shorthand escapes provide shorthands for certain commonly-used character classes:

\d [[:digit:]]
\s [[:space:]]
\w [[:alnum:]_] (note underscore)
\D [^[:digit:]]
\S [^[:space:]]
\W [^[:alnum:]_] (note underscore)

Within bracket expressions, '\d', '\s', and '\w' lose their outer brackets, and '\D', '\S', and '\W' are illegal. (So, for example, [a-c\d] is equivalent to [a-c[:digit:]]. Also, [a-c\D], which is equivalent to [a-c^[:digit:]], is illegal.)

A constraint escape is a constraint, matching the empty string if specific conditions are met, written as an escape:

\A matches only at the beginning of the string
\m matches only at the beginning of a word
\M matches only at the end of a word
\y matches only at the beginning or end of a word
\Y matches only at a point that is not the beginning or end of a word
\Z matches only at the end of the string
\m (where m is a nonzero digit) a back reference, see below
\mnn (where m is a nonzero digit, and nn is some more digits, and the decimal value mnn is not greater than the number of closing capturing parentheses seen so far) a back reference, see below

A word is defined as in the specification of [[:<:]] and [[:>:]] above. Constraint escapes are illegal within bracket expressions.

A back reference matches the same string matched by the parenthesized subexpression specified by the number, so that (e.g.) ([bc])\1 matches bb or cc but not 'bc'. The subexpression must entirely precede the back reference in the RE. Subexpressions are numbered in the order of their leading parentheses. Non-capturing parentheses do not define subexpressions.

There is an inherent historical ambiguity between octal character-entry escapes and back references, which is resolved by heuristics, as hinted at above. A leading zero always indicates an octal escape. A single non-zero digit, not followed by another digit, is always taken as a back reference. A multi-digit sequence not starting with a zero is taken as a back reference if it comes after a suitable subexpression (i.e. the number is in the legal range for a back reference), and otherwise is taken as octal.


Regular Expression Character Names

Note that the character names are case sensitive.

NUL '\0'
SOH '\001'
STX '\002'
ETX '\003'
EOT '\004'
ENQ '\005'
ACK '\006'
BEL '\007'
alert '\007'
BS '\010'
backspace '\b'
HT '\011'
tab '\t'
LF '\012'
newline '\n'
VT '\013'
vertical-tab '\v'
FF '\014'
form-feed '\f'
CR '\015'
carriage-return '\r'
SO '\016'
SI '\017'
DLE '\020'
DC1 '\021'
DC2 '\022'
DC3 '\023'
DC4 '\024'
NAK '\025'
SYN '\026'
ETB '\027'
CAN '\030'
EM '\031'
SUB '\032'
ESC '\033'
IS4 '\034'
FS '\034'
IS3 '\035'
GS '\035'
IS2 '\036'
RS '\036'
IS1 '\037'
US '\037'
space ' '
exclamation-mark '!'
quotation-mark '"'
number-sign '#'
dollar-sign '$'
percent-sign '%'
ampersand '&'
apostrophe '\''
left-parenthesis '('
right-parenthesis ')'
asterisk '*'
plus-sign '+'
comma ','
hyphen '-'
hyphen-minus '-'
period '.'
full-stop '.'
slash '/'
solidus '/'
zero '0'
one '1'
two '2'
three '3'
four '4'
five '5'
six '6'
seven '7'
eight '8'
nine '9'
colon ':'
semicolon ';'
less-than-sign '<'
equals-sign '='
greater-than-sign '>'
question-mark '?'
commercial-at '@'
left-square-bracket '['
backslash '\'
reverse-solidus '\'
right-square-bracket ']'
circumflex '^'
circumflex-accent '^'
underscore '_'
low-line '_'
grave-accent '''
left-brace '{'
left-curly-bracket '{'
vertical-line '|'
right-brace '}'
right-curly-bracket '}'
tilde '~'
DEL '\177'