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5.1 Overview of regular expression in sed

To know how to use sed, people should understand regular expressions (regexp for short). A regular expression is a pattern that is matched against a subject string from left to right. Most characters are ordinary: they stand for themselves in a pattern, and match the corresponding characters. Regular expressions in sed are specified between two slashes.

The following command prints lines containing the word ‘hello’:

sed -n '/hello/p'

The above example is equivalent to this grep command:

grep 'hello'

The power of regular expressions comes from the ability to include alternatives and repetitions in the pattern. These are encoded in the pattern by the use of special characters, which do not stand for themselves but instead are interpreted in some special way.

The character ^ (caret) in a regular expression matches the beginning of the line. The character . (dot) matches any single character. The following sed command matches and prints lines which start with the letter ‘b’, followed by any single character, followed by the letter ‘d’:

$ printf "%s\n" abode bad bed bit bid byte body | sed -n '/^b.d/p'

The following sections explain the meaning and usage of special characters in regular expressions.