How to use a case-insensitive modifier in a textbox regular expression?

I'd like to use the case-insensitive modifier (/i) in the regular expression, under Validation, for a single line textbox in a form.

For example, if I were attempting to validate a URL that begins with http://...  I want to allow these variations:

http://

hTtP://

HTtp://

and so on.  I realize that I can accomplish this with brackets, as in [Hh][Tt][Tt][Pp]://, but it gets more cumbersome as the pattern match gets longer.

Adding the modifier to the end of the regex, like I would in Javascript, breaks it.  The only hits I have found while searching are about setting the Ignore Case switch in a regex action.