问题描述
在测试另一个用户的问题的答案时，我发现了一些我不知道的东西理解.问题是用单个空格替换字符串中的所有文字\t \n \r字符.
When testing an answer for another user's question I found something I don't understand. The problem was to replace all literal \t \n \r characters from a string with a single space.
现在，我尝试的第一个模式是:
Now, the first pattern I tried was:
/(?:\\[trn])+/
令人惊讶的是没有用.我在Perl中尝试了相同的模式，但效果很好.经过一番尝试和错误后，我发现PHP希望该模式匹配3或4个反斜杠，如下所示:
which surprisingly didn't work. I tried the same pattern in Perl and it worked fine. After some trial and error I found that PHP wants 3 or 4 backslashes for that pattern to match, as in:
/(?:\\\\[trn])+/
或
/(?:\\\[trn])+/
这些模式-令我惊讶的是-两者都有效.为什么这些多余的反斜杠是必需的?
these patterns - to my surprise - both work. Why are these extra backslashes necessary?
推荐答案
您需要4个反斜杠来表示正则表达式中的1个，因为:
You need 4 backslashes to represent 1 in regex because:
- 2个反斜杠用于对字符串(
"\\\\" -> \\)进行转义 - 1个反斜杠用于在正则表达式引擎(
\\ -> \)中进行转义
- 2 backslashes are used for unescaping in a string (
"\\\\" -> \\) - 1 backslash is used for unescaping in the regex engine (
\\ -> \)
在PHP文档中，
因此\\\[，
- 1个反斜杠用于对
\进行转义，因为\[无效("\\\[" -> \\[) - 1个反斜杠用于在正则表达式引擎(
\\[ -> \[)中进行转义
- 1 backslash is used for unescaping the
\, one stay because\[is invalid ("\\\[" -> \\[) - 1 backslash is used for unescaping in the regex engine (
\\[ -> \[)
是的，但是不是一个好习惯.
Yes it works, but not a good practice.
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