G'day lirc201,

Welcome to the Monastery.

You've missed some information which could be important. Is there a minimum number of characters? Filenames can't start with [_.-], but can they end with all, some or none of those?

Just this week, I implemented something along these lines for production code. The requirements were: names could be just one character long; the start and end characters (the same character for one-character names) must match [A-Za-z0-9]; the middle characters for names with three or more characters must match [A-Za-z0-9_.-]. The regex for this:

qr{\A[A-Za-z0-9](?:[A-Za-z0-9_.-]*?[A-Za-z0-9]|)\z}
[download]

Note that, in a bracketed character class, '.' is not special and '-' is only special when between two characters to form a range: as you can see, you don't actually need to escape any characters.

Here's a limited test:

$ perl -E '
    my @x = (qw{A AA AAA _ __ ___ -A A- A.A A. .A A-A A_A}, "A\n", "A\
+tA");
    my $re = qr{\A[A-Za-z0-9](?:[A-Za-z0-9_.-]*?[A-Za-z0-9]|)\z};
    say "|$_| is ", /$re/ ? "OK" : "BAD" for @x
'
|A| is OK
|AA| is OK
|AAA| is OK
|_| is BAD
|__| is BAD
|___| is BAD
|-A| is BAD
|A-| is BAD
|A.A| is OK
|A.| is BAD
|.A| is BAD
|A-A| is OK
|A_A| is OK
|A
| is BAD
|A      A| is BAD
[download]

Modify that to suit your own filename specifications. Add some more tests which should probably include digits and lowercase letters.