How to use the xregexp.execLb function in xregexp

function execRegex (text, pattern, start) {
  var results = null

  if (pattern.xregexp) {
    // a regex that xregexp can handle right out of the gate.
    results = xregexp.exec(text, pattern, start)
  } else if (/^\(\?[><][=!]?/.exec(pattern.original)) {
    // a leading lookbehind regex.
    results = xregexp.execLb(text, pattern.original, start)
  } else if (/\(\?[><][=!]?/.exec(pattern.original)) {
    // allow for an alternation chracter followed by
    // a lookbehind regex.
    var splitPattern = pattern.original.split(/\|(\(\?[><][=!][^|]*)/g)
    if (splitPattern.length > 1) results = alternationPrefixedLookbehinds(text, splitPattern, start)
  }

  return results
}

// rebuild valid regex from splitting on (foo|(?<=foo)).
  for (var i = 0, pattern; (pattern = splitPattern[i]) !== undefined; i++) {
    if (/\(\?[><][=!]?/.exec(pattern)) {
      patterns.push(currentPattern)
      currentPattern = ''
    }
    currentPattern += pattern
  }
  patterns.push(currentPattern)

  // now apply each pattern.
  for (i = 0, pattern; (pattern = patterns[i]) !== undefined; i++) {
    try {
      if (/\(\?[><][=!]?/.exec(pattern)) {
        result = xregexp.execLb(text, pattern, start)
      } else {
        result = xregexp.exec(text, xregexp(pattern), start)
      }
      if (result) return result
    } catch (e) {
      // we're officially in uncharted territory.
      return null
    }
  }

  return null
}