badeball/xpath-lexer

var lexer = new RegExp([
  "\\$?(?:(?![0-9-])[\\w-]+:)?(?![0-9-])[\\w-]+",
  "\\d+\\.\\d+",
  "\\.\\d+",
  "\\d+",
  "\\/\\/",
  "/",
  "\\.\\.",
  "\\.",
  "\\s+",
  "::",
  ",",
  "@",
  "-",
  "=",
  "!=",
  "<=",
  "<",
  ">=",
  ">",
  "\\|",
  "\\+",
  "\\*",
  "\\(",
  "\\)",
  "\\[",
  "\\]",
  "\"[^\"]*\"",
  "'[^']*'"
].join("|"), "g");

export default class XPathLexer {
  private tokens: { value: string, position: number }[];
  private index: number;

  constructor(expression: string) {
    var match = expression.match(lexer);

    if (match === null) {
      throw new Error("Invalid character at position 0");
    }

    if (match.join("") !== expression) {
      var position = 0;

      for (var i = 0; i < match.length; i++) {
        if (expression.indexOf(match[i]) !== position) {
          break;
        }

        position += match[i].length;
      }

      throw new Error("Invalid character at position " + position);
    }

    this.tokens = match.map(function (token) {
      return {
        value: token,
        position: 0
      };
    });

    this.tokens.reduce(function (previousToken, nextToken) {
      nextToken.position = previousToken.position + previousToken.value.length;
      return nextToken;
    });

    this.tokens = this.tokens.filter(function (token) {
      return !/^\s+$/.test(token.value);
    });

    this.index = 0;
  }

  length() {
    return this.tokens.length;
  }

  next() {
    if (this.index === this.tokens.length) {
      return null;
    } else {
      var token = this.tokens[this.index++];

      return token && token.value;
    }
  }

  back() {
    if (this.index > 0) {
      this.index--;
    }
  }

  peak(n?: number) {
    var token = this.tokens[this.index + (n || 0)];

    return token && token.value;
  }

  empty() {
    return this.tokens.length <= this.index;
  }

  position() {
    if (this.index === this.tokens.length) {
      var lastToken = this.tokens[this.index - 1];

      return lastToken.position + lastToken.value.length;
    } else {
      return this.tokens[this.index].position;
    }
  }
}