Offering a girl friendship after I rejected her?

If I rejected a girl and she moved on (quickly) and later on I regret my decision and want her back but she's already taken, is it okay then to explain my situation to her and offer her friendship even though she has a boyfriend right now? I can't say that I'd feel ready to pursue her again, but I thought of the possibility that she might be single again some day.
Offering a girl friendship after I rejected her?
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