Why do look-ahead and look-behind have to be fixed-width patterns?

[inhahe]

Hi i'm a newbie at this and probably always will be, so don't be surprised
if I don't know what i'm talking about.

but I don't understand why regex look-behinds (and look-aheads) have to be
fixed-width patterns.

i'm getting the impression that it's supposed to make searching
exponentially slower otherwise

but i just don't see how.

say i have the expression (?<=.*?:.*?:).*
all the engine has to do is search for .*?:.*?:.*, and then in each result,
find .*?:.*?: and return the string starting at the point just after the
length of the match.
no exponential time there, and even that is probably more inefficient than
it has to be.