Javascript regular expression not working in Firefox

Question

This regular expression fails in Firefox but works in IE.

function validate(x) {
   return /.(jpeg|jpg)$/ig.test(x);
}

Can anybody explain why?

Answer 1

In regex expressions, "." by itself means "any character". Did you mean "\." to mean "period"?

Answer 2

The function is using a regular expression pattern .(jpeg|jpg)$ to test strings. It looks like the intent is to validate filenames to make sure they have an extension of either jpg or jpeg.

As James Bailey pointed out, there is an error in that the period should be escaped with a backslash. A period in a regular expression will match any character. So, as shown, the pattern would match both imagexjpg and image.jpg.

Answer 3

If you are testing with just the filename, then setting the 'g' flag for greedy doesn't make much sense since you are matching at the end of the string anyway - i ran the following:

function validate(x) {
  return /\.(jpeg|jpg)$/i.test(x);
}

var imagename = 'blah.jpg';
alert (validate(imagename));    // should be true
imagename = 'blah.jpeg';
alert (validate(imagename));    // should be true
imagename = 'blah.png';
alert (validate(imagename));    // should be false

all three tests came out as expected in FF.

As for 'how it works' - regular expressions can get quite tricky - but I will explain the details of the above pattern:

• the slash / is used to delimit the pattern
• the dot . has a special meaning of 'any non-whitespace character' - writing it as \. forces the patten to only match a fullstop
• the () mean a collection (it gets more complicated, but that covers this usage)
• the | means 'or'
• $ means the end of the string (or the end of a line in a multi-line text)
• the i after the second slash means 'case insensitive'
• the g means 'greedy' - which doesn't make much sense, so I removed it.

so..

/\.(jpeg|jpg)$/i

means "a string ending in either .jpeg or .jpg - ignoring case"

so... .JPEG, .jpg, .JpeG, etc... will all pass now...