A regular expression (regex or regexp) is a sequence of characters that specifies a search pattern. Usually such patterns are used by string-searching algorithms for "find" or "find and replace" operations on strings, or for input validation.
The below tables assists you on how to use different regular expressions:
Character
Explanation
.
Anything. Any character except newline
a
The character a
ab
The string ab
a|b
a or b
a*
0 or more a's
\
Escapes a special character
Character
Explanation
*
0 or more
+
1 or more
?
0 or 1
{2}
Exactly 2
{2, 5}
Between 2 and 5
{2,}
2 or more
Character
Explanation
(...)
Capturing group
(?P<Y>...)
Capturing group named Y
(?:...)
Non-capturing group
(?>...)
Atomic group
(?|...)
Duplicate group numbers
\Y
Match the Y'th captured group
(?P=Y)
Match the named group Y
(?R)
Recurse into entire pattern
(?Y)
Recurse into numbered group Y
(?&Y)
Recurse into named group Y
\g{Y}
Match the named or numbered group Y
\g<Y>
Recurse into named or numbered group Y
(?#...)
Comment
Character
Explanation
[ab-d]
One character of: a, b, c, d
[^ab-d]
One character except: a, b, c, d
[\b]
Backspace character
\d
One digit
\D
One non-digit
\s
One whitespace
\S
One non-whitespace
\w
One word character
\W
One non-word character
Character
Explanation
^
Start of string
\A
Start of string, ignores m flag
$
End of string
\Z
End of string, ignores m flag
\b
Word boundary
\B
Non-word boundary
\G
Start of match
(?=...)
Positive lookahead
(?!...)
Negative lookahead
(?<=...)
Positive lookbehind
(?<!...)
Negative lookbehind
(?()|)
Conditional
Character
Explanation
i
Ignore case
m
^ and $ match start and end of line
s
. matches newline as well
x
Allow spaces and comments
J
Duplicate group names allowed
U
Ungreedy quantifiers
(?iLmsux)
Set flags within regex
Character
Explanation
Newline
Carriage return
Tab
\0
Null character
\YYY
Octal character YYY
\xYY
Hexadecimal character YY
\x{YY}
Hexadecimeal character YY
\cY
Control character Y
Character
Explanation
[:alnum:]
Letters and digits
[:alpha:]
Letters
[:ascii:]
Ascii codes 0 - 127
[:blank:]
Space or tab only
[:cntrl:]
Control characters
[:digit:]
Decimal digits
[:graph:]
Visible characters, except space
[:lower:]
Lowercase letters
[:print:]
Visible characters
[:punct:]
Visible punctuation characters
[:space:]
Whitespace
[:upper:]
Uppercase letters
[:word:]
Word characters
[:xdigit:]
Hexadecimal digits