add yyless() ability to lexer (issue #84)

lib/jison/lexer.js

@@ -173,6 +173,10 @@ RegExpLexer.prototype = {
         this._more = true;
         return this;
     },
+    // retain first n characters of the match
+    less: function (n) {
+        this._input = this.match.slice(n) + this._input;
+    },
     // displays upcoming input, i.e. for error messages
     pastInput: function () {
         var past = this.matched.substr(0, this.matched.length - this.match.length);

tests/lexer/regexplexer.js

@@ -665,3 +665,19 @@ exports["test case insensitivity"] = function() {
 
     assert.equal(lexer.lex(), "CAT");
 };
+
+exports["test less"] = function() {
+    var dict = {
+        rules: [
+            ["cat", "this.less(2); return 'CAT';" ],
+            ["t", "return 'T';" ]
+        ],
+    };
+    var input = "cat";
+
+    var lexer = new RegExpLexer(dict);
+    lexer.setInput(input);
+
+    assert.equal(lexer.lex(), "CAT");
+    assert.equal(lexer.lex(), "T");
+};