Express route matching problem, how to write?

  node.js, question
/**
 * Match the following request url and output different sizes of a picture
 * /photo/s2hNrpLZQBPRGU9W639Bg9M26ZReQ2Hn_S.jpg
 * /photo/s2hNrpLZQBPRGU9W639Bg9M26ZReQ2Hn_M.jpg
 * /photo/s2hNrpLZQBPRGU9W639Bg9M26ZReQ2Hn_L.jpg
 * /photo/s2hNrpLZQBPRGU9W639Bg9M26ZReQ2Hn.jpg
 *
 * Note: URL suffix is the original file extension, therefore, this suffix is variable.  Such as:. gif, .png
 */
 
 app.get('/photo/:uuid(_L|_M|_S)?  . plus?'  , require('./routes/photo').index);

The above regular expression/photo/:uuid(_L|_M|_S)? Add?Do route matching, as if the matching is not successful, ask for help ~ ~ ~

Sf.gg link:http://segmentfault.com/q/1010000003968631
Cnode link:https://cnodejs.org/topic/5640020d8e90ab7c391e9f85

App.get('/photo/[a-zA-Z0-9] plus (_L|_M|_S)?  \.[a-z]{3}', xxx);