With the latest etherpad-lite (v1.5.6, git sha: 0a9c631):
[2015-05-02 11:23:25.156] [ERROR] console - SyntaxError: Invalid regular expression: /^\/p\/(.*)\/(?(?:([^\/]+?))){0,1}subscribe=(.*)\/?$/: Invalid group
at new RegExp (<anonymous>)
at pathtoRegexp (/home/etherpad/etherpad-lite/src/node_modules/express/node_modules/path-to-regexp/index.js:69:10)
at new Layer (/home/etherpad/etherpad-lite/src/node_modules/express/lib/router/layer.js:32:17)
at Function.proto.route (/home/etherpad/etherpad-lite/src/node_modules/express/lib/router/index.js:482:15)
at EventEmitter.app.(anonymous function) [as get] (/home/etherpad/etherpad-lite/src/node_modules/express/lib/application.js:464:30)
at Object.exports.registerRoute [as hook_fn] (/home/etherpad/etherpad-lite/node_modules/ep_email_notifications/index.js:11:12)
at normalizedhook (/home/etherpad/etherpad-lite/src/static/js/pluginfw/hooks.js:15:27)
at hookCallWrapper (/home/etherpad/etherpad-lite/src/static/js/pluginfw/hooks.js:21:14)
at /home/etherpad/etherpad-lite/src/static/js/pluginfw/hooks.js:76:14
at Function._.map._.collect (/home/etherpad/etherpad-lite/src/node_modules/underscore/underscore.js:172:24)
With the latest etherpad-lite (v1.5.6, git sha: 0a9c631):