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15.6 What Gets Matched?

Regex usually matches strings according to the “leftmost longest” rule; that is, it chooses the longest of the leftmost matches. This does not mean that for a regular expression containing subexpressions that it simply chooses the longest match for each subexpression, left to right; the overall match must also be the longest possible one.

For example, ‘(ac*)(c*d[ac]*)\1’ matches ‘acdacaaa’, not ‘acdac’, as it would if it were to choose the longest match for the first subexpression.