Regular Expressions for Filters

A regular expression is a text model formed of ordinary characters (for example, letters from a to z) and special characters known as metacharacters. The model describes one or several strings that coincide when searching a text body. The regular expression is used as a template to collate a model of characters with the string being searched.

The table below includes a complete list of metacharacters and their behavior in the context of the regular expressions:

Metacharacters

Expression

Matches

X

The character x

\

The character \

[abc]

a, b, or c

[^abc]

Any character except a, b, or c (negation)

[A-Z]

From A to Z inclusive

.

Any character

^

Line start

$

Line end

X?

X, once or never

X*

X, zero or more times

X+

X, once or more times

XY

X followed by Y

X|Y

X or Y

(X)

X, as a group

Groups are enumerated by counting the brackets open from left to right. The zero group refers to the complete expression.

The \ character can be used to escape the metacharacters used in the expressions. For instance, the \\ expression represents a single \, and \( represents a parenthesis.