grep

A favorite amongst shell programmers, it can be a convenient tool. It is both portable and fast, and a number of command-line options make it a powerful ally.

For portability, avoid using egrep and/or grep -E. This chart can help you find a portable syntax to replace extended syntax.

Element	Extended syntax	Portable syntax
Grouping	( and )	\( and \)
Quantity 1 or more	+ or {1,}	\+ or \{1,\}
Quantity 0 or 1	? or {0,1}	\? or \{0,1\}
Quantity N	{N}	\{N\}
Quantity N or less	{,N}	\{,N\}
OR	∣	\∣
Word Bounding	\< and \> (same)	\< and \> (same)

From the GNU grep(1) manual, an example of an extended regular expression would be:

egrep '19|20|25' calendar

The portable syntax would be:

grep '19\|20\|25' calendar

You can search for more than one regular expression using the -e pattern option of grep. For example:

1 #!/bin/sh
2 printf 'a\nb\n' | grep -e a -e b

Produces:

a
b

This allows you to quickly search for one or more regular expressions. A match on any given regular expression will cause the line to be printed.