Regular Expressions

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

.	Any character except newline	\c	Control character
\d	Digit	\D	non Digit
\s	Whitespace	\S	non Whitespace
\w	Word character [A-Za-z0-9]	\W	non Word character
^	Start of string	$	End of string
*	0 or more matches of previous expression	( )	Subexpression
+	1 or more matches of previous expression	[ ]	Match anything between the [ ]. ^as first character negates the match
?	0 or 1 matches of previous expression. Stop search as soon as next expression is found (non greedy)

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

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