Rewrite rules are rules which apply to a string of characters, and replace some of these characters with another string. Chomsky's proposal for linguistics entailed that a grammar be described in terms of a finite number of rewrite rules capable of generating all and only grammatical sentences of a given language. 

  

 For example, E -> TF (where E is an expression, T is a term and F is a factor) is a rewrite rule (in the context free grammar for a RegularExpression.) 

  

  

 mod_rewrite uses a rule-based rewriting engine (based on a regular-expression parser) to rewrite requested URLs on the fly. It supports an unlimited number of rules and an unlimited number of attached rule conditions for each rule to provide a really flexible and powerful URL manipulation mechanism. The URL manipulations can depend on various tests, for instance server variables, environment variables, HTTP headers, time stamps and even external database lookups in various formats can be used to achieve a really granular URL matching. 

  

 "The great thing about mod_rewrite is it gives you all the configurability and flexibility of Sendmail. The downside to mod_rewrite is that it gives you all the configurability and flexibility of Sendmail." -- Brian Behlendorf, [Apache] Group