php - 需要正则表达式的解释

Question

谁能帮我解释一下这个正则表达式的含义吗？

$html = preg_replace("# <(?![/a-z]) | (?<=\s)>(?![a-z]) #exi", "htmlentities('$0')", $html);

有人将其添加到 How to strip tags in a safer way than using strip_tags function?但我无法理解。

这是我在 stackoverflow 上的第一篇文章，所以如果我犯了任何错误，请原谅我。

谢谢!

Answer 1

#...#      the # and # are just characters to start en end a REGEX
           (you can use a lot of character for this)
#exi       the e, x and i flags. See the PHP.net site for information
           about it

<          the < character
(?!...)    a negative lookahead. The REGEX matches when the characters
           after this are NOT equal to one of those
[/a-z]     a character class, matches for the / character and the
           letters a - z
|          OR
(?<=\s)    a positive lookbehind. The REGEX maches when there is
           \s (whitepspace) before
>          the > character
(?![a-z])  negative lookahead for the letters a - z

所以基本上，它匹配所有 <和>不用作标签的字符。例如，<foo和</foo与 foo> 不匹配也不会。但是1 < 3会匹配。这将被传递到 htmlentities功能并成为1 < 3 。现在，您可以省钱地使用 strip_tags仅删除标签。