Editing Regular expression (section)

====Metacharacters in POSIX extended====
The meaning of metacharacters [[escape character|escaped]] with a backslash is reversed for some characters in the POSIX Extended Regular Expression ('''ERE''') syntax. With this syntax, a backslash causes the metacharacter to be treated as a literal character. So, for example, <code>\(&nbsp;\)</code> is now <code>(&nbsp;)</code> and <code>\{&nbsp;\}</code> is now <code>{&nbsp;}</code>. Additionally, support is removed for <code>\''n''</code> backreferences and the following metacharacters are added:

{| class="wikitable"
|-
! Metacharacter
! Description
|- valign="top"
! <code>?</code>
| Matches the preceding element zero or one time. For example, <code>ab?c</code> matches only "ac" or "abc".
|-
! <code>+</code>
| Matches the preceding element one or more times. For example, <code>ab+c</code> matches "abc", "abbc", "abbbc", and so on, but not "ac".
|-
! <code><nowiki>|</nowiki></code>
| The choice (also known as alternation or set union) operator matches either the expression before or the expression after the operator. For example, <code><nowiki>abc|def</nowiki></code> matches "abc" or "def".
|}

'''Examples:'''
* <code>[hc]?at</code> matches "at", "hat", and "cat".
* <code>[hc]*at</code> matches "at", "hat", "cat", "hhat", "chat", "hcat", "cchchat", and so on.
* <code>[hc]+at</code> matches "hat", "cat", "hhat", "chat", "hcat", "cchchat", and so on, but not "at".
* <code>cat|dog</code> matches "cat" or "dog".

POSIX Extended Regular Expressions can often be used with modern Unix utilities by including the [[command line]] flag <var>-E</var>.