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    <p>I don't think regular expressions are going to help you much here. They might be able to do some of the really simple matching, but what you're really looking at here is a grammar parsing problem. You might want to read up on languages designed to express abstract grammars, like <a href="http://en.wikipedia.org/wiki/Extended_Backus%E2%80%93Naur_Form" rel="nofollow">Extended Backus-Naur Form</a> (EBNF). It sounds intimidating, but it's really not that hard to grasp. Once you're able to describe your grammar in a formal language, suddenly parsing it becomes much easier (at the very least, you have a specification of what kind of inputs are valid). For example, you might have the following EBNF for your problem:</p> <pre><code>expression = "every" time-unit|time-unit-list|composite-time-unit time-unit = { ordinal } "day" | "weekday" ordinal = "first" | "second" | "third" | ... </code></pre> <p>And so on. This is not a trivial job; parsing an English sentence, even a fairly restrictive one like this can be quite involved. However, it is a well-established and rigorous method.</p> <p>Once you've got your grammar defined, you can build a parser for it. This is a matter of looking for <em>terminals</em> (like "every") and then matching them to a rule. For example, you might have something like the following (pseudocode):</p> <pre><code>words = split(/\s*/,lowercase(input)) if( words[0] == "every" ) { switch( words[1] ) { case "first": case "second": case "third": ... parseTimeUnit(words); break; case "day": everyDay = true; break; ... } } </code></pre> <p>Depending on the complexity of your grammar, you might look into automatically generating the parser with something like <a href="http://en.wikipedia.org/wiki/Yacc" rel="nofollow">Yacc</a>.</p> <p>You've bitten yourself off a hunk of a problem, but it's a rewarding one to work through, so good luck!</p> <p><strong>Update</strong>: I only suggested Yacc because it's among the oldest parser generators I know of. However, there are a million of them, and a lot of them will emit Javascript for you. You can check out <a href="http://en.wikipedia.org/wiki/Comparison_of_parser_generators" rel="nofollow">Wikipedia's comparison of parser generators</a> for more information.</p>
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