Simple apache nginx parser with NodeJS

parser.js

var patterns = {
    '[': '\\[([^\\]]+)\\]',
    '![': '[^\\[]+',
    '"':  '"([^"]+)"',
    '!"': '[^"]+',
    'default': '(\\S+)', // default pattern
    '!default': '\\s+'
};

function Parser(format) {
    var self = this;
    self.format = format;
    self.position = [];

    self.pattern = self.format.split(' ').reduce(function(pre, cur, index) {
        var key, variable = cur;
        key = patterns[cur[0]] ?
            (variable = variable.slice(1, -1), cur[0]) :
            'default';

        pre += (index === 0 ? '^' : patterns['!' + key]) + patterns[key];
        self.position[index] = variable;
        return pre;
    }, '', self);

    self.regexp = new RegExp(self.pattern);
}

Parser.prototype.match = function(text) {
    return text.match(this.regexp).slice(1);
};

Parser.prototype.parse = function(text) {
    var log = {};
    this.match(text).forEach(function(match, index) {
        log[this.position[index]] = match;
    }, this);
    return log;
};

module.exports = function(pattern) {
    return new Parser(pattern);
};

request.txt

[nginx.example.com] xxx.xxx.xxx.xxx - - [13/Sep/2012:11:24:12 +0800] 0.022 6083127088 "POST /search/ HTTP/1.1" 200 547 "http://nt.example.com/category/all" "Mozilla/5.0 (Windows NT 5.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/33.0.1750.117 Safari/537.36" "key=value; key2=value2" nt.example.com xx.xx.xx.xxx:80 0.022 http

test.js

var fs = require('fs');
var debug = require('debug')('test:parser');
var createParser = require('../lib/parser');

describe('parser', function() {
    var fields = ['remote_addr', 'remote_user', 'time_local', 'request_time',
        'connection', 'request', 'status', 'body_bytes_sent', 'http_referer',
        'http_user_agent', 'http_cookie', 'host', 'upstream_addr',
        'upstream_response_time'
    ];
    var format = '[remote_addr] remote_user [time_local] request_time connection "request" status body_bytes_sent "http_referer" "http_user_agent" "http_cookie" host upstream_addr upstream_response_time';
    var sample = fs.readFileSync(__dirname + '/request.txt', 'utf8');
    var parser;

    beforeEach(function() {
        parser = createParser(format);
    });

    it('should create pattern', function() {
        debug('pattern', parser.pattern);
        debug('regexp', parser.regexp);
        parser.should.have.properties('pattern', 'regexp');
    });

    it('should create placeholder variable position map', function() {
        debug('position', parser.position);
        parser.should.have.property('position');
    });

    it('should match group', function() {
        var matches = parser.match(sample);
        debug('matches', matches);
        matches.length.should.equal(fields.length);
    });

    it('should parse nginx log format', function() {
        var log = parser.parse(sample);
        debug('log', log);
        log.should.have.properties(fields);
    });
});