String comparision

I have a string like ab or abc of whatever length. But i want to know whether another string ( for example, abcfghijkl, OR a<space> bcfghijkl ab<space> cfghijkl OR a<space>bcfghijkl OR ab<space> c<space> fghijkl ) starts with ab or abc... space might existing on the longer string... If so, i wanna remove the ab or abc from the longer string and get the rest of it. Can you please guide me how to do this..

Thanks in advance!

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