Chapter 4. Regular expressions

In this chapter we discuss:

Using regular expressions
Regular expression metacharacters
Finding patterns in files or output

4.1. Regular expressions

4.1.1. What are regular expressions?

A regular expression is a pattern that describes a set of strings. Regular expressions are constructed analogously to arithmetic expressions by using various operators to combine smaller expressions.

The fundamental building blocks are the regular expressions that match a single character. Most characters, including all letters and digits, are regular expressions that match themselves. Any metacharacter with special meaning may be quoted by preceding it with a backslash.

4.1.2. Regular expression metacharacters

A regular expression may be followed by one of several repetition operators (metacharacters):

Table 4-1. Regular expression operators

Operator	Effect
.	Matches any single character.
?	The preceding item is optional and will be matched, at most, once.
*	The preceding item will be matched zero or more times.
+	The preceding item will be matched one or more times.
{N}	The preceding item is matched exactly N times.
{N,}	The preceding item is matched N or more times.
{N,M}	The preceding item is matched at least N times, but not more than M times.
-	represents the range if it's not first or last in a list or the ending point of a range in a list.
^	Matches the empty string at the beginning of a line; also represents the characters not in the range of a list.
$	Matches the empty string at the end of a line.
\b	Matches the empty string at the edge of a word.
\B	Matches the empty string provided it's not at the edge of a word.
\<	Match the empty string at the beginning of word.
\>	Match the empty string at the end of word.

Two regular expressions may be concatenated; the resulting regular expression matches any string formed by concatenating two substrings that respectively match the concatenated subexpressions.

Two regular expressions may be joined by the infix operator "|"; the resulting regular expression matches any string matching either subexpression.

Repetition takes precedence over concatenation, which in turn takes precedence over alternation. A whole subexpression may be enclosed in parentheses to override these precedence rules.

4.1.3. Basic versus extended regular expressions

In basic regular expressions the metacharacters "?", "+", "{", "|", "(", and ")" lose their special meaning; instead use the backslashed versions "\?", "\+", "\{", "\|", "$", and "$".

Check in your system documentation whether commands using regular expressions support extended expressions.