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What Are Regular Expressions?
Simply put, regular expressions are symbolic notations used to identify patterns in text. In some ways, they resemble the shell’s wildcard method of matching file and pathnames, but on a much grander scale. Regular expressions are supported by many command line tools and by most programming languages to facilitate the solution of text manipulation problems. However, to further confuse things, not all regular expressions are the same; they vary slightly from tool to tool and from programming language to language. For our discussion, we will limit ourselves to regular expressions as described in the POSIX stan- dard (which will cover most of the command line tools), as opposed to many program- ming languages (most notably Perl), which use slightly larger and richer sets of notations.