Bash-Beginners-Guide

Chapter 4. Regular expressions
In this chapter we discuss:
♦ Using regular expressions
♦ Regular expression metacharacters
♦ Finding patterns in files or output
♦ Character ranges and classes in Bash
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}
{N,}
{N,M}
The preceding item is matched exactly N times.
The preceding item is matched N or more times.
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.
Chapter 4. Regular expressions 56