php – pcre.backtrack_limit的“单位”是什么？

/* First check that we haven't called match() too many times,or that we
haven't exceeded the recursive call limit. */

if (md->match_call_count++ >= md->match_limit) RRETURN(PCRE_ERROR_MATCHLIMIT);

这被转换为“PHP_PCRE_BACKTRACK_LIMIT_ERROR”错误here.

根据pcreapi手册页(见https://serverfault.com/a/408272/140833)：

Internally,PCRE uses a function called match() which it calls
repeatedly (sometimes recursively). The limit set by match_limit is
imposed on the number of times this function is called during a match,
which has the effect of limiting the amount of backtracking that can
take place. For patterns that are not anchored,the count restarts
from zero for each position in the subject string.

我认为该单位因此类似于“回溯尝试次数”.虽然我不确定它是1对1.

这是一个用简单的“Catastrophic Backtracking”正则表达式隔离错误案例的演示：

<?php

ini_set('pcre.backtrack_limit',100);

for ($len = 1000; $len <= 1001; $len++) {

    $x = str_repeat("x",$len);
    $ret = preg_match("/x+x+y/",$x);

    echo "len = " . $len . "n";
    echo "preg_match = " . $ret . "n";
    echo "PREG_BACKTRACK_LIMIT_ERROR = " . PREG_BACKTRACK_LIMIT_ERROR . "n";
    echo "preg_last_error = " . preg_last_error() . "n";
    echo "n";
}

在此运行此代码：https://3v4l.org/EpaNC,以获得此输出：

len = 1000
preg_match = 0
PREG_BACKTRACK_LIMIT_ERROR = 2
preg_last_error = 0

len = 1001
preg_match = 
PREG_BACKTRACK_LIMIT_ERROR = 2
preg_last_error = 2