Extra backslash needed in PHP regexp pattern

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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.

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