Regular Expressions

References

• Regular-Expressions.info, The Premier website about Regular Expressions
• Regular expression on Wikipedia

Engines

Powerful engines:

• Perl
• PCRE

Open source regex engine implemented into PHP for instance

Less powerful engines:

• sed

Use Extended regular expressions (switch -r) so that meta-characters (){} have their special meaning when unquoted.

• grep

Character Classes

Class	Meaning	Comment
[ae]	Matches a or e
[a-z]	Matches any char in range a...z
[^a-z]	Matches any char not in range a...z
\d	Digit - Equivalent to [0-9]
\w	Word character - Equivalent to [A-Za-z0-9_]
\s	Whitespace character - Equivalent to [ \t\r\n]
\D	Negated \d, i.e. [^\d]
\W	Negated \w, i.e. [^\w]
\S	Negated \s, i.e. [^\s]

About negated class, note that [\D\S] is not the same as [^\d\s]. The latter will not match a character that is either a digit or a whitespace. The former will match any character that is either not a digit, or not a whitespace, i.e. it will match any character...

Zero-length matches

The regex here are zero-length, meaning they match a zero-length string, either because they match particular positions in the string (such as end-of-line or beginning-of-line anchors), or because the matched string is dropped after evaluation (like assertions, which only yield a boolean value, match or not matched).

Anchors:

Anchor	Meaning	Comment
^	Beginning-of-line anchor
$	End-of-line anchor
\b	Word boundary anchor
\B	Negated word boundary anchor
\<	Start-of-a-word anchor	GNU extensions
\>	End-of-a-word anchor	GNU extensions

Assertions:

Assertion	Meaning	Comment
(?=regex)	Lookahead positive assertion	e.g. \b(?=\w{0,3}cat)\w{6}\b, matches locate
(?!regex)	Lookahead negative assertion	e.g. \b(?!\w{0,3}cat)\w{6}\b, matches relica but not locate
(?<=regex)	Lookbehind positive assertion
(?<!regex)	Lookbehind negative assertion

Examples

Sed - The list below is actually for Extended regular expression (switch -r).

Regexp	Description
.	Match any character
gray|grey	Match gray or grey
gr(a|e)y	Match gray or grey
gr[ae]y	Match gray or grey
file[^0-2]	Match file3 or file4, but not file0, file1, file2.
colou?r	(zero or one) - Match Color or Colour.
ab*c	(zero or more) - Match ac, abc, abbc, ....
ab+c	(one or more) - Match abc, abbc, abbbc, ....
a{3,5}	(at least m and not more than n times) - Match aaa, aaaa, aaaaa.
^on single line$	(start and end of line) - Match on single line on a single line.