lib/less/parser/parser.js
function (untilTokens) {
                var i, e, done, value, 
                    tok = untilTokens || ';',
                    index = parserInput.i, result = [];

                function testCurrentChar() {
                    var char = parserInput.currentChar();
                    if (typeof tok === 'string') {
                        return char === tok;
                    } else {
                        return tok.test(char);
                    }
                }
                if (testCurrentChar()) {
                    return;
                }
                value = [];
                do {
                    e = this.comment();
                    if (e) {
                        value.push(e);
                        continue;
                    }
                    e = this.entity();
                    if (e) {
                        value.push(e);
                    }
                } while (e);

                done = testCurrentChar();
                
                if (value.length > 0) {
                    value = new(tree.Expression)(value);
                    if (done) {
                        return value;
                    }
                    else {
                        result.push(value);
                    }
                    // Preserve space before $parseUntil as it will not
                    if (parserInput.prevChar() === ' ') {
                        result.push(new tree.Anonymous(' ', index));
                    }
                }
                parserInput.save();
                
                value = parserInput.$parseUntil(tok);

                if (value) {
                    if (typeof value === 'string') {
                        error('Expected \'' + value + '\'', 'Parse');
                    }
                    if (value.length === 1 && value[0] === ' ') {
                        parserInput.forget();
                        return new tree.Anonymous('', index);
                    }
                    var item;
                    for (i = 0; i < value.length; i++) {
                        item = value[i];
                        if (Array.isArray(item)) {
                            // Treat actual quotes as normal quoted values
                            result.push(new tree.Quoted(item[0], item[1], true, index, fileInfo));
                        }
                        else {
                            if (i === value.length - 1) {
                                item = item.trim();
                            }
                            // Treat like quoted values, but replace vars like unquoted expressions
                            var quote = new tree.Quoted('\'', item, true, index, fileInfo);
                            quote.variableRegex = /@([\w-]+)/g;
                            quote.propRegex = /\$([\w-]+)/g;
                            result.push(quote);
                        }
                    }
                    parserInput.forget();
                    return new tree.Expression(result, true);
                }
                parserInput.restore();
            }
function (untilTokens) {
                var i, e, done, value, 
                    tok = untilTokens || ';',
                    index = parserInput.i, result = [];

                function testCurrentChar() {
                    var char = parserInput.currentChar();
                    if (typeof tok === 'string') {
                        return char === tok;
                    } else {
                        return tok.test(char);
                    }
                }
                if (testCurrentChar()) {
                    return;
                }
                value = [];
                do {
                    e = this.comment();
                    if (e) {
                        value.push(e);
                        continue;
                    }
                    e = this.entity();
                    if (e) {
                        value.push(e);
                    }
                } while (e);

                done = testCurrentChar();
                
                if (value.length > 0) {
                    value = new(tree.Expression)(value);
                    if (done) {
                        return value;
                    }
                    else {
                        result.push(value);
                    }
                    // Preserve space before $parseUntil as it will not
                    if (parserInput.prevChar() === ' ') {
                        result.push(new tree.Anonymous(' ', index));
                    }
                }
                parserInput.save();
                
                value = parserInput.$parseUntil(tok);

                if (value) {
                    if (typeof value === 'string') {
                        error('Expected \'' + value + '\'', 'Parse');
                    }
                    if (value.length === 1 && value[0] === ' ') {
                        parserInput.forget();
                        return new tree.Anonymous('', index);
                    }
                    var item;
                    for (i = 0; i < value.length; i++) {
                        item = value[i];
                        if (Array.isArray(item)) {
                            // Treat actual quotes as normal quoted values
                            result.push(new tree.Quoted(item[0], item[1], true, index, fileInfo));
                        }
                        else {
                            if (i === value.length - 1) {
                                item = item.trim();
                            }
                            // Treat like quoted values, but replace vars like unquoted expressions
                            var quote = new tree.Quoted('\'', item, true, index, fileInfo);
                            quote.variableRegex = /@([\w-]+)/g;
                            quote.propRegex = /\$([\w-]+)/g;
                            result.push(quote);
                        }
                    }
                    parserInput.forget();
                    return new tree.Expression(result, true);
                }
                parserInput.restore();
            }
Used for custom properties, at-rules, and variables (as fallback)
Parses almost anything inside of {} [] () "" blocks
until it reaches outer-most tokens.

First, it will try to parse comments and entities to reach
the end. This is mostly like the Expression parser except no
math is allowed.