Regex-PHP中的反斜杠

Question

I am trying to learn Regex in PHP and stuck in here now. My ques may appear silly but pls do explain.
I went through a link:
Extra backslash needed in PHP regexp pattern

But I just could not understand something:

In the answer he mentions two statements:

2 backslashes are used for unescaping in a string ("\\\\" -> \\)
1 backslash is used for unescaping in the regex engine (\\ -> \)

My ques:
what does the word "unescaping" actually means? what is the purpose of unescaping? Why do we need 4 backslashes to include it in the regex?

Answer 1

The backslash has a special meaning in both regexen and PHP. In both cases it is used as an escape character. For example, if you want to write a literal quote character inside a PHP string literal, this won't work:

$str = ''';

PHP would get "confused" which ' ends the string and which is part of the string. That's where \ comes in:

$str = '\'';

It escapes the special meaning of ', so instead of terminating the string literal, it is now just a normal character in the string. There are more escape sequences like as well.

This now means that \ is a special character with a special meaning. To escape this conundrum when you want to write a literal \, you'll have to escape literal backslashes as \\:

$str = '\\'; // string literal representing one backslash

This works the same in both PHP and regexen. If you want to write a literal backslash in a regex, you have to write /\\/. Now, since you're writing your regexen as PHP strings, you need to double escape them:

$regex = '/\\\\/';

One pair of \\ is first reduced to one \ by the PHP string escaping mechanism, so the actual regex is /\\/, which is a regex which means "one backslash".