Introducing Regular Expressions

Appendix A. Regular Expression Reference

This appendix is a reference for regular expressions.

Regular Expressions in QED

QED (short for Quick Editor) was originally written for the Berkeley Time-Sharing System, which ran on the Scientific Data Systems SDS 940. A rewrite of the original QED editor by Ken Thompson for MIT’s Compatible Time-Sharing System yielded one of the earliest (if not the first) practical implementation of regular expressions in computing. Table A-1, taken from pages 3 and 4 of a 1970 Bell Labs memo, outlines the regex features in QED. It amazes me that most of this syntax has remained in use to this day, over 40 years later.

Table A-1. QED regular expressions

Feature	Description
literal	“a) An ordinary character [literal] is a regular expression which matches that character.”
^	“b) ^ is a regular expression which matches the null character at the beginning of a line.”
$	“c) $ is a regular expression which matches the null character before the character <nl> [newline] (usually at the end of a line).”
.	“d) . is a regular expression which matches any character except <nl> [newline].”
[<string>]	“e) “[<string>]” is a regular expression which matches any of the characters in the <string> and no others.”
[^<string>]	“f) “[^<string>] is a regular expression which matches any character but <nl> [newline] and the characters of the <string>.”
*	“g) A regular expression followed by “*” is a regular expression which matches any number (including zero) of adjacent occurrences of the text ...