Regular expression control characters
Control characters are metacharacters, operators, and replacement text characters that can be used in regular expressions.
Only half-width characters are recognized. Any full-width characters that correspond to the characters in the following tables are not recognized.
Character | Allowed outside of sets | Allowed inside of sets | Description |
---|---|---|---|
\a |
Yes | Yes | Match a BELL, \u0007 |
\A |
Yes | Match at the beginning of the input. Differs
from ^ in that \A does not match
after a new line within the input. |
|
\b |
Yes | Match if the current position is a word boundary.
Boundaries occur at the transitions between word (\w )
and non-word (\W ) characters, with combining marks
ignored. |
|
\B |
Yes | Match if the current position is not a word boundary. | |
\cX |
Yes | Yes | Match a control-X character. |
\d |
Yes | Yes | Match any character with the Unicode General
Category of Nd (Number, Decimal Digit.) |
\D |
Yes | Yes | Match any character that is not a decimal digit. |
\e |
Yes | Yes | Match an ESCAPE, \u001B |
\E |
Yes | Yes | Terminates a \Q ... \E quoted
sequence. |
\f |
Yes | Yes | Match a FORM FEED, \u000C . |
\G |
Yes | Match if the current position is at the end of the previous match. | |
\n |
Yes | Yes | Match a LINE FEED, \u000A |
\N{UNICODE CHARACTER NAME} |
Yes | Yes | Match the named character. |
\p{UNICODE PROPERTY NAME} |
Yes | Yes | Match any character with the specified Unicode Property. |
\P{UNICODE PROPERTY NAME} |
Yes | Yes | Match any character not having the specified Unicode Property. |
\Q |
Yes | Yes | Places quotation marks around all following
characters until \E |
\r |
Yes | Yes | Match a CARRIAGE RETURN, \u000D |
\s |
Yes | Yes | Match a white space character. White space is
defined as [\t\n\f\r\p{Z}] |
\S |
Yes | Yes | Match a non-white space character. |
\t |
Yes | Yes | Match a HORIZONTAL TABULATION, \u0009 |
\uhhhh |
Yes | Yes | Match the character with the hex value hhhh |
\Uhhhhhhhh |
Yes | Yes | Match the character with the hex value hhhhhhhh .
Exactly 8 hex digits must be provided, even though the largest Unicode
code point is \U0010ffff |
\w |
Yes | Yes | Match a word character. Word characters are
as follows:
|
\W |
Yes | Yes | Match a non-word character. |
\x{hhhh} |
Yes | Yes | Match the character with hex value hhhh .
From one to 6 hex digits can be supplied. |
\xhh |
Yes | Yes | Match the character with two-digit hex value hh |
\X |
Yes | Match a Grapheme Cluster. | |
\Z |
Yes | Match if the current position is at the end of input, but before the final line terminator, if one exists. | |
\z |
Yes | Match if the current position is at the end of input. | |
\n |
Yes | Back Reference. Match whatever the nth capturing
group matched. n must be a number > 1 and < total
number of capture groups in the pattern. |
|
\0ooo |
Yes | Yes | Match an Octal character. 'ooo' is
from one to three octal digits. 0377 is the largest
allowed Octal character. The leading zero is required; it distinguishes
Octal constants from back references. |
[pattern] |
Yes | Yes | Match any one character from the set. |
. |
Yes | Match any character. | |
^ |
Yes | Match at the beginning of a line. | |
$ |
Yes | Match at the end of a line. | |
\ |
Yes | Places quotation marks around the character
that follows. Characters that must have surrounding quotation marks
to be treated as literals are * ? + [ ( ) { } ^ $ | \ . / |
|
\ |
Yes | Places quotation marks around the character
that follows. Characters that must be quoted to be treated as literals
are [ ] \ Characters that might need to be quoted, depending on the context are - & |
Operator | Description |
---|---|
| |
Alternation. A|B matches either A or B |
* |
Match 0 or more times. Match as many times as possible. |
+ |
Match 1 or more times. Match as many times as possible. |
? |
Match zero or one time. Prefer one. |
{n} |
Match exactly n times. |
{n,} |
Match at least n times. Match
as many times as possible. |
{n,m} |
Match between n and m times.
Match as many times as possible, but not more than m . |
*? |
Match 0 or more times. Match as few times as possible. |
+? |
Match 1 or more times. Match as few times as possible. |
?? |
Match zero or one time. Prefer zero. |
{n}? |
Match exactly n times. |
{n,}? |
Match at least n times, but
no more than required for an overall pattern match. |
{n,m}? |
Match between n and m times.
Match as few times as possible, but not less than n |
*+ |
Match 0 or more times. Match as many times as possible when first encountered, do not retry with fewer even if overall match fails (Possessive Match) |
++ |
Match 1 or more times. Possessive match. |
?+ |
Match zero or 1 time. Possessive match. |
{n}+ |
Match exactly n times. |
{n,}+ |
Match at least n times. Possessive
Match. |
{n,m}+ |
Match between n and m times.
Possessive Match. |
( ... ) |
Capturing parentheses. Range of input that matched the parenthesized subexpression is available after the match. |
(?: ... ) |
Non-capturing parentheses. Groups the included pattern, but does not provide capturing of matching text. More efficient than capturing parentheses. |
(?> ... ) |
Atomic-match parentheses. First match of the
parenthesized subexpression is the only one tried. If it does not
lead to an overall pattern match, back up the search for a match to
a position before the "(?>" |
(?# ... ) |
Free-format comment (?# comment ) |
(?= ... ) |
Look-ahead assertion. True if the parenthesized pattern matches at the current input position, but does not advance the input position. |
(?! ... ) |
Negative look-ahead assertion. True if the parenthesized pattern does not match at the current input position. Does not advance the input position. |
(?<= ... ) |
Look-behind assertion. True if the parenthesized
pattern matches text that precedes the current input position. The
last character of the match is the input character just before the
current position. Does not alter the input position. The length of
possible strings that is matched by the look-behind pattern must not
be unbounded (no * or + operators.) |
(?<!...) |
Negative Look-behind assertion. True if the
parenthesized pattern does not match text that precedes preceding
the current input position. The last character of the match is the
input character just before the current position. Does not alter the
input position. The length of possible strings that is matched by
the look-behind pattern must not be unbounded (no * or + operators.) |
(?ismwx-ismwx: ... ) |
Flag settings. Evaluate the parenthesized expression with the specified flags enabled or disabled. |
(?ismx-ismx) |
Flag settings. Change the flag settings. Changes
apply to the portion of the pattern that follows the setting. For
example, (?i) changes to a not case-sensitive match. |
Example expression | Description |
---|---|
[abc] |
Match any of the characters a , b ,
or c |
[^abc] |
Negation - match any character except a , b ,
or c |
[A-M] |
Range - match any character from A to M .
The characters to include are determined by Unicode code point order. |
[\u0000-\U0010ffff] |
Range - match all characters. |
|
Characters with Unicode Category = Letter .
All forms that are shown are equivalent. |
[\P{Letter}] |
Negated property. (Uppercase \P )
Match everything except Letters. |
[\p{numeric_value=9}] |
Match all numbers with a numeric value of 9 .
Any Unicode Property might be used in set expressions. |
[\p{Letter}&&\p{script=cyrillic}] |
Logical AND or intersection. Match the set of all Cyrillic letters. |
[\p{Letter}--\p{script=latin}] |
Subtraction. Match all non-Latin letters. |
[[a-z][A-Z][0-9]] [a-zA-Z0-9]] |
Implicit Logical OR or Union of Sets. The examples match ASCII letters and digits. The two forms are equivalent. |
[:script=Greek:] |
Alternate POSIX-like syntax for properties.
Equivalent to \p{script=Greek} |