Regular Expression for File Name with No Space and a Single 3 Character Extension

[JiaPei Jen]

If we require a file name submitted by users must not have space and have a single 3 character extension (for example: .doc, .txt, .pdf)

I think the 3 characters extension may be something similar to ^\d{3}\d*$ (I can be wrong). And how do we set the 'no space' constraint?

[David Harkness]

You'll need to be more specific. Do you want to allow digits in the extension? What about non-alphanumeric characters in the filename? In any case, here's one that allows any non-space character in the base name and any alphanumeric (letter or number) character in the extension:

[^ ]+\.\w{3}

Note that this allows a newline (\n) and other symbol characters for the base name. You may want to limit this a bit to something like

[-\w_,!()]+\.\w{3}

[Layne Lund]

David,

Did you miss the ^ towards the beginning of your second regex?

[David Harkness]

Originally posted by Layne Lund:
Did you miss the ^ towards the beginning of your second regex?

No, the first regex excludes space while the second one includes a specific set of characters: a-z A-Z 0-9 - _ , ! ( and ). It all depends on how restrictive JiaPei wants to be with the two parts of the filename. For example, must it match Unix or Windows naming restrictions?

[Stan James]

I just stumbled across RegEx Coach the other day. It's a neat interactive way to play with expressions and see how they work. It won't write them for you but it will surely help you understand them. Anybody else have favorite tools in this space?

[Stefan Wagner]

No, but I want to mention, that ~ is an often used filename-part (for backups), and valid on dos and linux.

and I don't like '.jav', '.htm' and '.cla' - files.