Thank you, unfortunately that works only for simple strings, not regexes. Consider:

'bated' =~ /^bat{3}/; false
'bated' =~ /^bat/; true
[download]

I'm trying to look at the beginning of a string to know if it can potentially match a regex, without knowing the full string. Which is of course different from making a completely different regex with substr($regex.

(Edited for bad examples)