Special Characters in Regular Expressions

The following tables summarize the special characters in regular expressions:

Escape character

Special character Meaning
\ Escape character for special characters

Special character for single character strings

Special character Meaning
. Placeholder for any single character
\C Placeholder for any single character
\d Placeholder for any single digit
\D Placeholder for any character other than a digit
\l Placeholder for any lower-case letter
\L Placeholder for any character other than a lower-case letter
\s Placeholder for a blank character
\S Placeholder for any character other than a blank character
\u Placeholder for any upper-case letter
\U Placeholder for any character other than an upper-case letter
\w Placeholder for any alphanumeric character including _
\W Placeholder for any non-alphanumeric character except for _
[ ] Definition of a value set for single characters
[^ ] Negation of a value set for single characters
[ - ] Definition of a range in a value set for single characters
[ [:alnum:] ] Description of all alphanumeric characters in a value set
[ [:alpha:] ] Description of all letters in a value set
[ [:blank:] ] Description for blank characters and horizontal tabulators in a value set
[ [:cntrl:] ] Description of all control characters in a value set
[ [:digit:] ] Description of all digits in a value set
[ [:graph:] ] Description of all graphic special characters in a value set
[ [:lower:] ] Description of all lower-case letters in a value set
[ [:print:] ] Description of all displayable characters in a value set
[ [:punct:] ] Description of all punctuation characters in a value set
[ [:space:] ] Description of all blank characters, tabulators, and carriage feeds in a value set
[ [:unicode:] ] Description of all Unicode characters in a value set with a code larger than 255
[ [:upper:] ] Description of all upper-case letters in a value set
[ [:word:] ] Description of all alphanumeric characters in a value set, including _
[ [:xdigit:] ] Description of all hexadecimal digits in a value set
\a \f \n \r \t \v Diverse platform-specific control characters
[..] Reserved for later enhancements
[==] Reserved for later enhancements

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Special characters for character string patterns

Special character Meaning
{n} Concatenation of n single characters
{n,m} Concatenation of at least n and a maximum of m single characters
{n,m}? Reserved for later enhancements
? One or no single characters
* Concatenation of any number of single characters including 'no characters'
*? Reserved for later enhancements
+ Concatenation of any number of single characters excluding 'no characters'
+? Reserved for later enhancements
| Linking of two alternative expressions
( ) Definition of subgroups with registration
(?: ) Definition of subgroups without registration
\1, \2, \3 ... Placeholder for the register of subgroups
\Q ... \E Definition of a string of literal characters
(? ... ) Reserved for later enhancements

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Special characters for search strings

Special character Meaning
^ Anchor character for the start of a line
\A Anchor character for the start of a character string
$ Anchor character for the end of a line
\Z Anchor character for the end of a character string
\< Start of a word
\> End of a word
\b Start or end of a word
\B Space between characters within a word
(?= ) Preview condition
(?! ) Negated preview condition

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Special characters for replacement texts

Special character Meaning
$0, $& Placeholder for the whole found location
$1, $2, $3... Placeholder for the register of subgroups
$` Placeholder for the text before the found location
$' Placeholder for the text after the found location

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