Javascript regex for matching/extracting file extension

Question

The following regex

var patt1=/[0-9a-z]+$/i; 

extracts the file extension of strings such as

filename-jpg filename#gif filename.png 

How to modify this regular expression to only return an extension when string really is a filename with one dot as separator ? (Obviously filename#gif is not a regular filename)

UPDATE Based on tvanofsson's comments I would like to clarify that when the JS function receives the string, the string will already contain a filename without spaces without the dots and other special characters (it will actually be handled a slug). The problem was not in parsing filenames but in incorrectly parsing slugs - the function was returning an extension of "jpg" when it was given "filename-jpg" when it should really return null or empty string and it is this behaviour that needed to be corrected.

Answer 1

Just add a . to the regex

var patt1=/\.[0-9a-z]+$/i; 

Because the dot is a special character in regex you need to escape it to match it literally: \..

Your pattern will now match any string that ends with a dot followed by at least one character from [0-9a-z].

Example:

[   "foobar.a",   "foobar.txt",   "foobar.foobar1234" ].forEach( t =>    console.log(     t.match(/\.[0-9a-z]+$/i)[0]   )  )

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if you want to limit the extension to a certain amount of characters also, than you need to replace the +

var patt1=/\.[0-9a-z]{1,5}$/i; 

would allow at least 1 and at most 5 characters after the dot.