Appendix A. Regular Expressions

A. Regular Expressions

IN THIS APPENDIX

Characters	898
Delimiters	898
Simple Strings	898
Special Characters	898
Rules	901
Bracketing Expressions	902
The Replacement String	902
Extended Regular Expressions	903

A regular expression defines a set of one or more strings of characters. 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, vim, emacs, grep, awk, sed, and other utilities. The regular expressions used in shell ambiguous file references are different and are described in "Filename Generation/Pathname Expansion" on page 133.