Regular Expressions

Regular expressions or regexp are used to find strings in text.

Perl, PHP and Python support all in below table. Bash and SQL only support the POSIX part.

.	Any character except newline (POSIX)	\c	Control character (not for Python3)
\d	Digit	\D	non Digit
\s	Whitespace	\S	non Whitespace
\w	Word character [A-Za-z0-9]	\W	non Word character
^	Start of string (POSIX)	$	End of string (POSIX)
*	0 or more matches of previous expression (POSIX)	( )	Subexpression (POSIX)
+	1 or more matches of previous expression (POSIX)	[ ]	Match any of the characters between the [ ]. ^as first character negates the match (POSIX)
?	0 or 1 matches of previous expression. Stop search as soon as next expression is found (non greedy) (POSIX)	{x,y}	Match the previous expression between x an y times. When y is ommited match exactly x times (POSIX)
\n	Newline	\<int>	Refers to the <int>-th subexpression (POSIX)

Examples

/[^/]*$

Match last / and everything following

Perl

perl -lne 'print $1 if (/<regexp(subexp)>/)'

Commandline to print the first subexp in a match.

$var =~ /<pattern>/

Generic syntax, this expression is true if the pattern is matched in $var

@array = $var =~ m/<pattern>/g;

Put all matches (or all first submatches) of <pattern> in var into @array

Following variables are when a match is made:

$&

Contains the string matched by the last pattern match

$`

The string preceding whatever was matched by the last pattern match, not counting patterns matched in nested blocks that have been exited already.

$'

The string following whatever was matched by the last pattern match, not counting patterns matched in nested blocks that have been exited already.
Example:

    $_ = 'abcdefghi';
    /def/;
    print "$`:$&:$'";
    # prints abc:def:ghi

$1

String matched by the first subexpression.

$+

The last bracket matched by the last search pattern. This is useful if you don't know which of a set of alternative patterns matched.
Example:

    /Version: (.*)|Revision: (.*)/
    && ($rev = $+);

Python

Check Python:Strings#Regular_Expressions_(regexp) and/or the cheatography site