# RegExp Based Lexers

Chevrotain Lexers are defined using standard ECMAScript regular expressions. This means there is no need to learn a new syntax and/or semantics.

In addition existing JavaScript regExp libraries can be easily used, for example by using the awesome xRegExp library (opens new window) one can simplify the creation of complex patterns and avoid code duplication.

$.RULE("statement", () => {
  const fragments = {}

  // A utility to create re-usable fragments using xRegExp
  function FRAGMENT(name, def) {
    fragments[name] = XRegExp.build(def, fragments)
  }

  // a utility to create a pattern using previously defined fragments
  function MAKE_PATTERN(def, flags) {
    return XRegExp.build(def, fragments, flags)
  }

  // define fragments
  FRAGMENT("IntegerPart", "-?(0|[1-9][0-9]*)")
  FRAGMENT("FractionalPart", "\\.[0-9]+")
  FRAGMENT("ExponentPart", "[eE][+-]?[0-9]+")

  const IntValue = createToken({
    name: "IntValue",
    // Simple use case, not really needed in this case except for avoiding duplication.
    pattern: MAKE_PATTERN("{{IntegerPart}}")
  })

  const FloatValue = createToken({
    name: "FloatValue",
    pattern: MAKE_PATTERN(
      // This regExp would be very hard to read without "named fragments"
      "{{IntegerPart}}{{FractionalPart}}({{ExponentPart}})?|{{IntegerPart}}{{ExponentPart}}"
    )
  })
})

See full executable example (opens new window) as part of the graphQL example grammar.