File: sed.info,  Node: Regular Expressions Overview,  Next: BRE vs ERE,  Up: sed regular expressions

5.1 Overview of regular expression in ‘sed’
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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 string ‘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'
     bad
     bed
     bid
     body

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