19.1. A Brief Introduction to Regular Expressions

An expression is a string of characters. Those characters that have an interpretation above and beyond their literal meaning are called metacharacters. A quote symbol, for example, may denote speech by a person, ditto, or a meta-meaning for the symbols that follow. Regular Expressions are sets of characters and/or metacharacters that UNIX endows with special features. [1]

The main uses for Regular Expressions (REs) are text searches and string manipulation. An RE matches a single character or a set of characters (a substring or an entire string).

Sed, awk, and Perl, used as filters in scripts, take REs as arguments when "sifting" or transforming files or I/O streams. See Example A-7 and Example A-12 for illustrations of this.

"Sed & Awk", by Dougherty and Robbins gives a very complete and lucid treatment of REs (see the Bibliography).



The simplest type of Regular Expression is a character string that retains its literal meaning, not containing any metacharacters.


Since sed, awk, and grep process single lines, there will usually not be a newline to match. In those cases where there is a newline in a multiple line expression, the dot will match the newline.

sed -e 'N;s/.*/[&]/' << EOF   # Here Document
# [line1
# line2]


awk '{ $0=$1 "\n" $2; if (/line.1/) {print}}' << EOF
line 1
line 2
# line
# 1

# Thanks, S.C.

exit 0