XRegExp tokenizer and parser that builds a new RegExp to validate strings against a given format string

FormatStringRegExpBuilderExample.js

// Get XRegExp in all it's goodness and install native stuff
var XRegExp = require("xregexp");
XRegExp.install("natives");


///////////////////////////////////////////////////////////////////////////////
/// LEXER
///////////////////////////////////////////////////////////////////////////////

// Create a formatToken XRegExp to match against the formatStr
var formatTokenRE = XRegExp.build("({{$token}})", {
  $token: XRegExp.build("({{$user}})|({{$date}})|({{$literal}})", {
    $user: /%u/,
    $date: /%d/,
    $literal: /.+?/
  })
}, "g");


///////////////////////////////////////////////////////////////////////////////
/// PARSER
///////////////////////////////////////////////////////////////////////////////

// Build a lookup table of replacements for the tokens received that will go into the compound RegExp
var tokenREs = {
  $user: /[a-zA-Z'-]+?/,
  $date: /\d+-\d+-\d+/,
  $literal: function($token){
    return new XRegExp(XRegExp.escape($token));
  }
};

// parse and convert tokens in formatStr to the replacements from tokenREs to build a compound RegExp
function parse(formatStr){
  var compoundREs = [];
  XRegExp.forEach(formatStr, formatTokenRE, function(match){
    for(var key in tokenREs){
    if(key in match && match[key] && match[key].length > 0){
      var handler = tokenREs[key];
      compoundREs.push(typeof(handler) == "function" ? handler(match[key]) : handler);
    }
    }  
  });

  // Join compoundREs here
  return XRegExp.union(compoundREs, "", "g"); // or whatever; where separator=="" here
}


///////////////////////////////////////////////////////////////////////////////
/// TEST
///////////////////////////////////////////////////////////////////////////////

// Some test data
var formatStr = "Hello %u! Today is %d.";
var testStr = "Hello AwesomeUser! Today is 09-20-2012.";

// Parse to a new RegExp and test
var formatRE = parse(formatStr);
if(!formatRE.test(testStr)){
  throw new Error("Invalid input for the format string!");
}