From the PCRE source code, this error is returned when "match()" is called more than 1,000,000 times recursively:
/* 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);
That is converted into a "PHP_PCRE_BACKTRACK_LIMIT_ERROR" error here.
According to the pcreapi manpage (see 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.
I think that the unit is therefore something like "Number of backtracking attempts". I'm not sure that it's 1-to-1 with that though.
Here's a demo isolating the error case with a simple "Catastrophic Backtracking" regex:
<?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";
}
Run this code here: https://3v4l.org/EpaNC, to get this output:
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
