Regular Expressions

A regular expression is a powerful pattern matching tool. Regular expressions are supported in text processing utilities, text editors, and programming languages.

Basic regular expression

This is the oldest and simplest form. Characters with special meaning inlude:

.

matches any single character

[ ]

bracket expression – matches a single character from the enclosed set

within brackets a range is defined with an in-between -

within brackets a logical NOT is indicated with a leading ^

*

multiplier – matches zero or more occurrences of the (smallest) previous expression

^

if first, anchors pattern to beginning of input

$

if last, anchors pattern to end of input

\

removes the special meaning of the following single character

Extended regular expression

This newer form has additional characters with special meaning:

?

multiplier – matches zero or one occurrence of the (smallest) previous expression

+

multiplier – matches one or more occurrences of the (smallest) previous expression

{n}

multiplier – matches n occurrences of the (smallest) previous expression

{n,}

multiplier – matches n or more occurrences of the (smallest) previous expression

{n,m}

multiplier – matches between n and m (inclusive) occurrences of the (smallest) previous expression

( )

grouping – enclosed characters become a single expression

|

alternation – logical OR, inside ( )

Advanced regular expression

This form contains numerous extensions and is available in newer programming languages.

Character classes

These describe sets of characters by name and are used in many contexts. In regular expressions character classes can be used only within bracket expressions. A few examples:

[:alnum:]

alpha-numeric

[:alpha:]

alphabetic

[:digit:]

numerals (decimal)

[:lower:]

lowercase

[:punct:]

punctuation

[:space:]

whitespace

[:upper:]

uppercase

Escapes

In POSIX regular expressions a non-special character following a backslash is undefined. In some implementations these constructions are defined to represent non-printing characters, though the definitions vary between implementations. A few common examples:

\n

newline

\r

carriage return

\t

horizontal tab