问题
I have a code written by a previous programmer, a regex preg_match that is throwing a compilation error:
$regex_t = "/" . $op . "(?\\>[^" . $op . $cl . "]+)*" . $cl . "/s";
preg_match($regex_t, $text, $inner);
The warning that I receive is:
Warning: preg_match(): Compilation failed: unrecognized character after (? or (?- at offset 4
Also, I want to mention that at var_dump($regex_t) the value is:
string '/\{(?\>[^\{\}]+)*\}/s' (length=21)
回答1:
For a strange reason, the programmer has escaped the > (that is never needed).
Only these characters need to be escaped to obtain a literal character (outside a character class):
( ) ^ $ [ \ | . * + ?
{ # only in these cases: {n} {m,n} {m,}
# where m and n are integers
+ the pattern delimiter
Most of the time an escaped character that doesn't need to be escaped (or that does not have a special meaning like \b \w \d ...) is simply ignored by the regex engine. But it's not the case here, because (?> is a fixed sequence to open an atomic group, and the sequence (? is not allowed except for these cases:
- a non capturing group:
(?:...) - an atomic group:
(?>...) - an inline modifier:
(?i)(?-i) - a non capturing group with inline modifiers:
(?i:...)(?-i:...) - a lookaround:
(?=...)(?!...)(?<=...)(?<!...) - a branch reset group:
(?|...|...) - a conditional test:
(?(condition)...|...) - a reference to a subpattern:
(?1)(?-1)(?R) - a definition group:
(?(DEFINE)...) - a named subpattern:
(?<John>...)(?P<John>...)(?'John'...) - a reference to a named subpattern:
(?&John)(?P>John)
So (?\ is not recognized as one of these sequences, this is why you obtain an error.
Perhaps the programmer wanted to write an atomic group (?>...) (This is probably the case, because since the group is repeated, using an atomic group reduces the backtracking if the following subpattern fails) or has forgotten the : for a non capturing group (?:\\ as Avinash suggests it, but to obtain a literal backslash you need four backslashes in a double quoted string.
来源：https://stackoverflow.com/questions/28646475/warning-preg-match-compilation-failed-unrecognized-character-after-or