PHP preg_replace：帮我理解（？=：）
在另一个问题中，有以下几行：
$value='x-Cem-Date:Wed, 16 Dec 2009 15:42:28 GMT';
$value = preg_replace('/(^.+?)(?=:)/e', "strtolower('\\1')", $value);
// yields 'x-cem-date:Wed, 16 Dec 2009 15:42:28 GMT'
(?=:) 位表示搜索对于结肠来说，它必须这样做。但是，我不明白 ?= 的特定语法。那里究竟发生了什么？
In another question, there are the following lines:
$value='x-Cem-Date:Wed, 16 Dec 2009 15:42:28 GMT';
$value = preg_replace('/(^.+?)(?=:)/e', "strtolower('\\1')", $value);
// yields 'x-cem-date:Wed, 16 Dec 2009 15:42:28 GMT'
That (?=:) bit indicates a search for the colon, it has to. But, I don't understand that particular syntax, with the ?=. What exactly is going on there?
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这是一个正向预测。它查看特定的子表达式是否出现在该点之后。但它不会消耗比赛中的任何东西：
正如您可能注意到的，环视在替换文本时特别有用，因为您可以不需要将周围环境包含到替换文本中。例如，要将每个后面不跟有
u的q替换为qu，您可以这样做，但这会捕获以下字符，因为它是匹配并稍后重新插入。您还可以使用lookaround:，
其中仅匹配和替换q，因此不再需要在替换字符串中包含部分匹配。
That's a positive lookahead. It looks whether the particular subexpression occurs after that point. But it doesn't consume anything in the match:
As you may notice, lookaround is especially helpful when replacing text since you don't need to include the surrounding environment into the replacement text. For example, to replace every
qthat is not followed by auwithquyou can dobut this captures the following character because it's part of the match and re-inserts it later again. You can also use lookaround:
where only the q gets matched and replaced, so including part of the match in the replacement string isn't necessary anymore.