Regular expression to allow file with only .txt or no extension

Question

I need a regular expression that should allow files only with .txt or with no extension.

For example: c:\\document\\assignment\\test.txt or c:\\document\\assignment\\testing

This is what I have thus far:

^[a-zA-Z]:(\\\\[a-zA-Z0-9\\p{Space}[\\p{Punct}&&[^\\Q\\/:*?\"<>|\\E]]\\(\\)]+[^\\.])+(?=(\\.txt|\\.TXT))(\\.txt|\\.TXT)$

The above is working only for the .txt but not for files with no extension.

Answer 1

The regular expression requires the case-insensitive flag, and should work with most regex engine flavors.

^(?:[a-z]:|\\|\.{1,2})(?:\\[^<>:"/\\|?*.]+)+(?:\.txt)?$

REY

Explanation

The regular expression produces a match by matching the following sub patterns.

• Start of the string
• Find the start of a path: either a drive letter, a back-slash for a unc name, or one or two periods for a relative path
• At least one path item that starts with a backslash and follows basic conventions from Naming Files, Paths, and Namespaces .
• Optionally ending in .txt
• End of the string.

You can simplify the expression if you don't need unc or relative paths by changing to first capture group to just az . You can add more extensions by changing the last capture group. For example, (?:\\.(txt|exe|com)? would match .txt , .exe , or .com .