Extra backslash needed in PHP regexp pattern

Question

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

Answer 1

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.