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The Art of Unix Programming
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Unix Programming - Applying Minilanguages - Case Study: Regular Expressions

Case Study: Regular Expressions

A kind of specification that turns up repeatedly in Unix tools and scripting languages is the regular expression (‘regexp’ for short). We consider it here as a declarative minilanguage for describing text patterns; it is often embedded in other minilanguages. Regexps are so ubiquitous that the are hardly thought of as a minilanguage, but they replace what would otherwise be huge volumes of code implementing different (and incompatible) search capabilities.

This introduction skates over some details like POSIX extensions, back-references, and internationalization features; for a more complete treatment, see Mastering Regular Expressions [Friedl].

Regular expressions describe patterns that may either match or fail to match against strings. The simplest regular-expression tool is grep(1), a filter that passes through to its output every line in its input matching a specified regexp. Regexp notation is summarized in Table8.1.

There are a number of minor variants of regexp notation:

Now that we've looked at some motivating examples, Table8.2 is a summary of the standard regular-expression wildcards. Note: we're not including the glob variant in this table, so a value of “All” implies only all three of the basic, extended/Emacs, and Perl/Python variants.[81]

Design practice in new languages with regexp support has stabilized on the Perl/Python variant. It is more transparent than the others, notably because backlash before a non-alphanumeric character always means that character as a literal, so there is much less confusion about how to quote elements of regexps.

Regular expressions are an extreme example of how concise a minilanguage can be. Simple regular expressions express recognition behavior that would otherwise have to be implenented with hundreds of lines of fussy, bug-prone code.


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The Art of Unix Programming
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