Test your code with this string (has a foo and is less than 50 chars so should match): 

$_ = "foo\n Oops";
print /^(?=.{0,50}$).*foo/i ? "Matches!" : $_;

[download]

This fixes your regex: 

/(?=^.{0,50}\z).*foo/si

[download]

\z only matches the end of the string (unlike $ which will match an embedded \n) and /s lets . match everything, including \n. Strictly you don't need the \z in this context and could leave the $ but it is good to know the difference. As you want to match foo it is more efficient to move the ^ .... \z into the lookahead which removes the need for the .* Death to dot star! Oops, updated per jehuni's comment below

This is a really silly way to do it that fulfils your criteria of being a single regex :-) 

$str =~ s/(foo)/&do_stuff($str) if length $1 < 50; $1/e;

[download]

cheers

tachyon

s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print

In reply to Re: validating string length with a regular expression by tachyon
in thread validating string length with a regular expression by jehuni

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