css.js

/**
 * Module dependencies.
 */

var fs = require('fs');

var css = fs.readFileSync('style.css', 'utf8');

function scan(buf) {
  var state = ['root'];

  state.name = function(){
    return this[this.length - 1];
  };

  return next;

  function o(pattern) {
    var captures;
    if ('string' == typeof pattern) {
      if (0 == buf.indexOf(pattern)) {
        buf = buf.slice(pattern.length);
        return pattern;
      }
    } else if (pattern instanceof RegExp) {
      if (captures = pattern.exec(buf)) {
        buf = buf.slice(captures[0].length);
        return captures[1];
      }
    }
  }

  function next() {
    return ident()
      || block()
      || colon()
      || eol()
      || string()
      || whitespace()
      || commentOpen()
      || commentClose()
      || comment();
  }

  function string() {
    var ret = o(/^"([^"]+)"/);
    if (!ret) return;
    return ['string', ret];
  }

  function whitespace() {
    if (o(/^(\s+)/)) return next();
  }

  function colon() {
    if (o(':')) return ['colon'];
  }

  function eol() {
    if (o(';')) return ['eol'];
  }

  function block() {
    if (o('{')) return ['block-open'];
    if (o('}')) return ['block-close'];
  }

  function ident() {
    var ret = o(/^([-\w]+)/);
    if (!ret) return;
    return ['ident', ret];
  }

  function comment() {
    if ('comment' != state.name()) return;
    var i = buf.indexOf('*/')
      , ret = buf.slice(0, i);
    buf = buf.slice(i);
    return ['comment', ret];
  }

  function commentClose() {
    var ret = o('*/');
    if (!ret) return;
    state.pop();
    return ['comment-close'];
  }

  function commentOpen() {
    var ret = o('/*');
    if (!ret) return;
    state.push('comment');
    return ['comment-open'];
  }
}

var next = scan(css);
var tok;
while (tok = next()) console.log(tok);

out.js

[ 'ident', 'body' ]
[ 'block-open' ]
[ 'ident', 'padding' ]
[ 'colon' ]
[ 'ident', '50px' ]
[ 'eol' ]
[ 'ident', 'font-size' ]
[ 'colon' ]
[ 'ident', '12px' ]
[ 'eol' ]
[ 'block-close' ]