Custom Token Patterns

Chevrotain is not limited to only using JavaScript regular expressions to define Tokens. Tokens can also be defined using arbitrary JavaScript code, for example:

// our custom matcher
function matchInteger(text, startOffset) {
  let endOffset = startOffset;
  let charCode = text.charCodeAt(endOffset);
  // 0-9 digits
  while (charCode >= 48 && charCode <= 57) {
    charCode = text.charCodeAt(endOffset);

  // No match, must return null to conform with the RegExp.prototype.exec signature
  if (endOffset === startOffset) {
    return null;
  } else {
    let matchedString = text.substring(startOffset, endOffset);
    // according to the RegExp.prototype.exec API the first item in the returned array must be the whole matched string.
    return [matchedString];

const IntegerToken = createToken({
  name: "IntegerToken",
  pattern: matchInteger,

This feature is often used to implement complex lexing logic, such as python indentationopen in new window.

See in depth guide for further details.