带有可选组的正则表达式多行模式会跳过有效数据 [英] Regex multiline mode with optional group skip valid data

问题描述

考虑下一个示例:

$payload = '
ababaaabbb =%=
ababaaabbb =%=
ababaa     =%=
';

$pattern = '/^[ab]+\s*(?:=%=)?$/m';
preg_match_all($pattern, $payload, $matches);
var_dump($matches);

预期的和实际的匹配结果是:

Expected and actual result of match is:

"ababaaabbb =%="
"ababaaabbb =%="
"ababaa     =%="

但是如果$payload更改为

$payload = '
ababaaabbb =%=
ababaaabbb =%=
ababaa     =%'; // "=" sign removed at EOL

实际结果是

"ababaaabbb =%="
"ababaaabbb =%="

但预期是

"ababaaabbb =%="
"ababaaabbb =%="
"ababaa     "

为什么会这样?由于?，组(?:=%=)?是可选的，并且有效载荷中的最后一个字符串也应出现在匹配结果中.

Why this happen? Group (?:=%=)? is optional due to ? and last string in payload should be also present in match results.

推荐答案

看看您当前的正则表达式图:

Have a look at your current regex graph:

=%= 是是可选的(请参阅white space和End of line分支之间的分支方式)，但是EOL 是必需的.这意味着在一个或多个a或b符号后，以及零个或多个空格之后，必须发生EOL .但是，您在第三行上有=% =>没有匹配.

The =%= is optional (see how the branch between white space and End of line forks), but the EOL is required. That means after one or more a or b symbols, and zero or more whitespaces, EOL must occur. However, you have =% on your 3rd line => NO MATCH.

现在，当您将$锚点移动到可选组中时:

Now, when you move the $ anchor into the optional group:

行尾现在也是可选，并且匹配1+ a或b字符和可选空格后，将返回匹配项.

The end of line is now optional, too, and the match will be returned after matching 1+ a or b chars and optional whitespaces.

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