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Top 100 Network Security Tools Computing Dictionary Sculpting text with regex, grep, sed and awk Theory: Regular languages Many tools for searching and sculpting text rely on a pattern language known as regular expressions. The theory of regular languages underpins regular expressions. (Caveat: Some modern "regular" expression systems can describe irregular languages, which is why the term "regex" is preferred for these systems.) Regular languages are a class of formal language equivalent in power to those recognized by deterministic finite automata (DFAs) and nondeterministic finite automata (NFAs). [See my post on converting regular expressions to NFAs.] In formal language theory, a language is a set of strings. For example, {"foo"} and {"foo", "foobar"} are formal (if small) languages. (Mathematicians don't typically put quotes around a string, preferring to let the fixed-width typewriter font distinguish it as one, but I'm guessing that programmers are more comfortable with the quotes around strings.) In regular language theory, there are two atomic languages: Useful grep flags The +? #!