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Linux Regular Expression

Regular expression is also called regex or regexp. It is a very powerful tool in Linux. Regular expression is a pattern for a matching string that follows some pattern.

Regex can be used in a variety of programs like grep, sed, vi, bash, rename and many more.

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Regular Expression Metacharacters

A regular expression may have one or several repeating metacharacters.

Metacharacter	Description
.	Replaces any character.
^	Matches start of string and represents characters not in the string.
$	Matches end of string.
*	Matches zero or more times the preceding character.
	Represents the group of characters.
()	Groups regular expressions.
?	Matches exactly one character.
+	Matches one or more times the preceding character.
{N}	Preceding character is matched exactly N times.
{N,}	Preceding character is matched exactly N times or more.
{N,M}	Preceding character is matched exactly N times, but not more than N times.
–	Represents the range.
b	Matches empty string at the edge of a word.
B	Matches empty string if it is not at the edge of a word.
<	Matches empty string at the beginning of a word.
>	Matches empty string at the end of a word.

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Regex Versions

There are three versions of regular expressions syntax:

• BRE : Basic Regular Expressions
• ERE : Extended Regular Expressions
• PRCE: Perl Regular Expressions

Depending on tool or programs, one or more of these versions can be used.

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