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 an operating system 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 string or a part of a string.


Some versions of sed, ed, and ex support escaped versions of the extended Regular Expressions described above, as do the GNU utilities.

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-13 and Example A-18 for illustrations of this.

The standard reference on this complex topic is Friedl's "Mastering Regular Expressions." "Sed & Awk," by Dougherty and Robbins also gives a very lucid treatment of REs. See the Bibliography for more information on these books.



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