3.1.1. Regular expressions on the left side of "s///"

All versions of sed support Basic Regular Expressions (BREs). For the syntax of BREs, enter "man ed" at a Unix shell prompt. A technical description of BREs from IEEE POSIX 1003.1-2001 and the Single UNIX Specification Version 3 is available online at: https://www.opengroup.org/onlinepubs/007904975/basedefs/xbd_chap09.html#tag_09_03

Sed normally supports BREs plus '\n' to match a newline in the pattern space, plus '\xREx' as equivalent to '/RE/', where 'x' is any character other than a newline or another backslash.

Some versions of sed support supersets of BREs, or "extended regular expressions", which offer additional metacharacters for increased flexibility. For additional information on extended REs in GNU sed, see sections 3.7 ("GNU/POSIX extensions to regular expressions") and 6.7.3 ("Special syntax in REs"), below.

Though not required by BREs, some versions of sed support \t to represent a TAB, \r for carriage return, \xHH for direct entry of hex codes, and so forth. Other versions of sed do not.

ssed (super-sed) introduced many new features for LHS pattern matching, too many to give here. The complete list is found in section 6.7.3.H ("ssed"), below.