Extra backslash needed in PHP regexp pattern

You need 4 backslashes to represent 1 in regex because:

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

From the PHP doc,

escaping any other character will result in the backslash being printed too¹

Hence for \[,

• 1 backslash is used for unescaping the , one stay because [ is invalid ("\[" -> \[)
• 1 backslash is used for unescaping in the regex engine (\[ -> [)

Yes it works, but not a good practice.

There are couple of problems:

1. Your regex pattern will also match an input of more than 15 characters.
2. Your regex will also other non-allowed characters in the middle like @ or # due to use of S

You can fix it by using a negative lookahead to disallow consecutive occurrence of period/hyphen/underscore and remove S from middle of regex that allows any non-space character

^[a-zA-Z0-9](?!.*[_.-]{2})[w.-]{4,13}[a-zA-Z0-9]$

RegEx Demo

Instead of meddling with preg_replace(), consider using var_export() to print a "true copy" of the string:

foreach ($strings as $s) {
    echo var_export($s, true), PHP_EOL;
}

Output:

'this is a simple string'
'Arnold once said: "I'll be back"'
'You deleted C:\*.*?'
'You deleted C:\*.*?'
'This will not expand: \n a newline'
'Variables do not $expand $either'

As you can see, sentence 3 and 4 are identical to PHP.