I'm not really sure if I understand what you mean. Of course if "the first half of your expression" matches, then the second one won't. Do you really need that alternation? Wouldn't you better split it in two separate regexen? Alternatively, isn't it that you really want

/^(?:$begin1|$begin2)(\d+)/
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instead?

Also, you seem to be familiar with regexen so that I may well be wrong, but your use of dots is somewhat suspect, thus I dare to ask... are you aware that "." matches "any charachter"?