Description:
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When creating a high-performance lexer (for example, an HTML5 parser), it is worthwhile to try to avoid the number of string copies made.  You can perform matches using offsets into your master source string.  However, preg_match will copy a substring for the entire matched region ($matches[0]) as well as for all captured patterns.  This can get expensive if the matched region/captured patterns are very large.

It would be helpful if PHP's preg_match* functions offered a flag, say PREG_LENGTH_CAPTURE, which returned the numeric length instead of the string.  In combination, PREG_OFFSET_CAPTURE|PREG_LENGTH_CAPTURE would return the numeric length in element 0 and the offset in element 1, and avoid the need to copy the matched substring unnecessarily.