Yes. The regex engine uses backtracking and marks its location whenever it has to make a decision. If the first decision doesn't work out, it backtracks to the last marked point and uses the next possible decision. The ? regex operator is such a decision point and I guess that basically, the two possibilities for a match are:

[012345]\d

[download]
and 

\d

[download]
So in this case, it would not get a match when trying /[012345]\d/ against 5, so it backtracks, and tries /\d/, which works.

In reply to Re^5: simple pattern match ? by Corion
in thread simple pattern match ? by Anonymous Monk

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