How to determine the static part of a regular expression at the beginning?

Is there a safe way to determine that part of an regular expression that is static i.e. that matches only one string?

I have a regular expression for file paths and I want to extract that part at the beginning that doesn't require regular expression so that I can perform a direct search to speed up performance.

For example, I have /some/path/.*\.jpg

Now I want /some/path/ and .*\.jpg separately.

EDIT

The pattern could have any valid form. Maybe it doesn't even have a static part at the beginning but for the most times it does.