lib/less/parser/parser.js
function () {
                    var value, index = parserInput.i;

                    parserInput.autoCommentAbsorb = false;

                    if (!parserInput.$str('url(')) {
                        parserInput.autoCommentAbsorb = true;
                        return;
                    }

                    value = this.quoted() || this.variable() || this.property() ||
                            parserInput.$re(/^(?:(?:\\[\(\)'"])|[^\(\)'"])+/) || '';

                    parserInput.autoCommentAbsorb = true;

                    expectChar(')');

                    return new(tree.URL)((value.value != null || 
                        value instanceof tree.Variable || 
                        value instanceof tree.Property) ?
                        value : new(tree.Anonymous)(value, index), index, fileInfo);
                }
function () {
                    var value, index = parserInput.i;

                    parserInput.autoCommentAbsorb = false;

                    if (!parserInput.$str('url(')) {
                        parserInput.autoCommentAbsorb = true;
                        return;
                    }

                    value = this.quoted() || this.variable() || this.property() ||
                            parserInput.$re(/^(?:(?:\\[\(\)'"])|[^\(\)'"])+/) || '';

                    parserInput.autoCommentAbsorb = true;

                    expectChar(')');

                    return new(tree.URL)((value.value != null || 
                        value instanceof tree.Variable || 
                        value instanceof tree.Property) ?
                        value : new(tree.Anonymous)(value, index), index, fileInfo);
                }
function ( ) { var value , index = parserInput . i ; parserInput . autoCommentAbsorb = false ; if ( ! parserInput . $str ( 'url(' ) ) { parserInput . autoCommentAbsorb = true ; return ; } value = this . quoted ( ) || this . variable ( ) || this . property ( ) || parserInput . $re ( / ^(?:(?:\\[\(\)'"])|[^\(\)'"])+ / ) || '' ; parserInput . autoCommentAbsorb = true ; expectChar ( ')' ) ; return new ( tree . URL ) ( ( value . value != null || value instanceof tree . Variable || value instanceof tree . Property ) ? value : new ( tree . Anonymous ) ( value , index ) , index , fileInfo ) ; }
Parse url() tokens  We use a specific rule for urls, because they don't really behave like standard function calls. The difference is that the argument doesn't have to be enclosed within a string, so it can't be parsed as an Expression.
Parse url () tokens We use a specific rule for urls because they don t really behave like standard function calls . The difference is that the argument doesn t have to be enclosed within a string so it can t be parsed as an Expression .