Question

I have been doing some searching for a regex that can be used as a rule to disallow users entering windows file paths without escaping the “\”. So far I have found this expression

[^\\]*$

However, this fails for the following:

C:\\Program Files\\testing

By fails I mean that it does not validate this string. Any help would be greatly appreciated, and yes I am bound to using regex.

Answer 1

^(\\\\|[^\\])*$

will match strings that only contain escaped \ characters or non-\ characters. (For a little extra performance, you could improve it to: ^(?:\\\\|[^\\]+)*$)

In Perl:

if ($subject =~ m/^(?:\\\\|[^\\]+)*$/) {
    # Successful match
} else {
    # Match attempt failed
}

This will match

C:\\Program Files\\test
abcd
h983475iuh 87435v z 87tr8v74
\\\\\\\\\\

and fail

C:\Program Files\test
\
\\\

etc.