为什么preg_match匹配最后一个子模式而不是第一个？

Question

I am trying to match the first hexadecimal address from a line that can contain many hexadecimal addresses, but instead I get the last.

My request is:

preg_match('%.*(0x[0-9a-f]{8}){1}.*%', $v, $current_match);

where the $v is a string like:

Line: 2 libdispatch.dylib 0x36eaed55 0x36eae000 + 3413

I would want to get 0x36eaed55, but my regular expression for $current_match[1] returns 0x36eae000 instead.

According to php documentation: $matches[1] will have the text that matched the first captured parenthesized subpattern, and so on.

Answer 1

The problem is that the * quantifier is greedy by default, so the first .* matches as much as possible while still allowing the entire expression to match. In this case, it means that .* will "gobble up" all of the hexadecimal constants but the last one, as (0x[0-9a-f]{8}){1} still needs to match.

One solution is to use the non-greedy operator *?. The first constant is found when using the following:

preg_match('%.*?(0x[0-9a-f]{8}){1}.*?%', $v, $current_match);

However, because you know that $v includes a hexadecimal constant, and you want the first one, then why not simply match against the pattern of the hexadecimal constant?

preg_match('%0x[0-9a-f]{8}%', $v, $current_match);

Even if you wanted the second, third, fourth, ... hexadecimal constant, you could use preg_match_all() with the same pattern:

preg_match_all('%0x[0-9a-f]{8}%', $v, $all_matches, PREG_PATTERN_ORDER);