Use of regular expression in Express.js Routing Pattern:

Routing is way of determining how an application responds to a client request to a particular endpoint.


Route definition has following structure:

    app.METHOD(PATH, HANDLER)

Here

    app is an instance of express.
    METHOD is an HTTP request method, in lowercase.(get,post,put,delete)
    PATH is a path on the server.This route paths can be strings, string patterns, or regular expressions.
    HANDLER is the function executed when the route is matched.

Consider following simple example
            app.get('/', function (req, res) {
                res.send('Hello World!')
            })
   here
            function (req, res) {
                res.send('Hello World!')
            }
    is handler.'get' is http method & '/' is path on server to match with.


Each route can have one or more handler functions, which are executed when the route is matched.

a) consider following routing
    router.get('/ab?cd', function (req, res) {
      res.send('ab?cd')
    })

    '?' in regular expression stands for once or none.Means 'b' can occurs once or will not occur at all at specified place

    this route will match with following URL
        1) http://localhost:3000/abcd
        2) http://localhost:3000/acd
  
    will not match
        1)http://localhost:3000/abccd
        2)http://localhost:3000/abbd

b) consider following routing
        router.get('/pq+rs', function (req, res) {
            res.send('pq+rs')
        })

    '+' in regular expression stands for atleast once or more,means q can occurs atleast once or more.

    this route will match with following url
        1)http://localhost:3000/pqrs
        2)http://localhost:3000/pqqrs
        3)http://localhost:3000/pqqqrs
  
    will not match with
        1) http://localhost:3000/prs
        2) http://localhost:3000/pqrrs
        3) http://localhost:3000/pqers
        4) http://localhost:3000/pqs


c) consider following route      

        router.get('/lm*no', function (req, res) {
            res.send('lm*no')
        })

    '*' stands for zero or more.so '*' can be replaced by any valid character group or not replaced at all.

        matches
            1) http://localhost:3000/lmno
            2) http://localhost:3000/lmxyzno

        will not match
            1)http://localhost:3000/lmo  
            2)http://localhost:3000/lno

d) consider following route

    router.get('/xy(zw)?e', function (req, res) {
        res.send('xy(zw)?e')
    })
 
    this says character group 'zw' can occur once or never.

    matches
        1) http://localhost:3000/xyzwe
        2) http://localhost:3000/xye

    will not match
        1) http://localhost:3000/xyzwzwe

e) we can pass parameter from PATH.if we prepend ':' then express knows its should be treated at URL parameter.All such URL param are available in req.params  collection.
      
        router.get('/movie/:movieId/song/:songId', function (req, res) {
            res.send(req.params)
        })

    Request:
        http://localhost:3000/movie/Dilwale%20Dulhania%20Le%20Jayenge/song/ruk%20ja%20o%20dil%20diwane
    Response:
        {
            "movieId":"Dilwale Dulhania Le Jayenge",
            "songId":"ruk ja o dil diwane"
        }


Lets add following route just above our route defined in section a.

router.get('/abcd', function (req, res,next) {
  console.log('abcd');
  next();
})      

Now hit http://localhost:3000/abcd

you will observe that on console 'abcd' got printed and on browser seeing 'ab?cd' so it gone through both routes it is matching.order depends upon how which route come first.

Now lets see below route here two function are acting as handler in single route

        router.get('/movietime', function (req, res, next) {
                console.log('sms movie time')
                next()
        }, function (req, res) {
                res.send('email movie time')
        })

        hit http://localhost:3000/movietime
        on console you can see
                sms movie time
        on browser you can see
            email movie time

        here both function got run in order they placed in route.      

References:      
    https://expressjs.com/en/guide/routing.html