How can I validate a Perl regex in user input?

I'm getting a list of inputs from the user of supposely valid perl regexp values. Examples could be:

\b[Bb]anana\b
\s*Apples[BANANA]\s+

Is there a safe way to validate these strings?

Solution

First, consider how much you want to let users do with a pattern. A Perl regex can run arbitrary code.

But, to validate that you can use a string as a pattern without it causing a fatal error, you can use the qr// operator to compile the string and return the regex. If there's a problem, the qr gives you a fatal error that you can catch with eval:

my $pattern = eval { qr/$input/ };

If you get back undef, the pattern was not valid. And, despite the comments in the question, there are infinite ways to make invalid patterns. I know because I type them in by hand all the time and I haven't run out of ways to mess up :)

This does not apply the pattern to a string, but you can use $pattern to make the match:

if( $pattern ) {
    $target =~ $pattern;  # or $target =~ m/$pattern/
    }