Backslash in Regex- PHP -

I'm trying to learn Reggae in PHP and now I'm stuck here. My questions may look silly, but to explain pls.
I went through a link:

But I could not understand:

In response, he mentioned two statements:

2 backslashes are used for not leaving a string ( "\\\\" -> \\ )
1 backslash Regex is not used to leave the engine ( \\ -> \ )

My question:
what is the word Does "not leave" really mean it? What is the purpose of unacking? Why do we need 4 backslashes to be included in regex?

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Backslash does not have a special meaning in both Regexen and PHP in both cases It is used as an Escape character. For example, if you want to type literal quotation character within a PHP string, then it will not work:

  $ str = '' ';   
 PHP is "confused" which ends with the  ' string and which is the part of the string. That is where  \  comes in:  
  $ str = '\' ';   
  exits   ' has special meaning, therefore instead the string is literally finished, now it's just a normal character string. There are also more Escape series like  \ n .  
 This means that  \  is a special character with special meaning. To avoid this puzzle when you want to write a literal  \ , you will have to protect the literal backslash from  \\ :  
  $ Str = '\\'; // string literally represents a backslash   
 This works similar to both PHP and regexen. If you want to write a literal backslash in a regedx, you will see the  / \ / . Now, since you are writing your regexen as a PHP string, you have to save it:  
  $ regex = '/ \\\\ /'; A pair of   
  \\  becomes the first in a  \  PHP string escaping mechanism, so the actual regex  / \\ / , which is a regex which means "a backslash"