IN THIS APPENDIX
The Replacement String
Extended Regular Expressions
A regular expression defines a set of one or more strings of characters. Several of the GNU/Linux utilities, including vi, emacs, grep, gawk, and sed, use regular expressions to search for and replace strings. A simple string of characters is a regular expression that defines one string of characters: itself. A more complex regular expression uses letters, numbers, and special characters to define many different strings of characters. A regular expression is said to match any string it defines.
This appendix describes the regular expressions used by ed, vi, emacs,