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Editorial Team
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Asked: 2026-05-13T12:37:49+00:00

When testing an answer for another user’s question I found something I don’t understand.

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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 characters from a string with a single space.

Now, the first pattern I tried was:

/(?:\\[trn])+/

which surprisingly didn’t work. I tried the same pattern in Perl and it worked fine. After some trial and error I found that PHP wants 3 or 4 backslashes for that pattern to match, as in:

/(?:\\\\[trn])+/

or

/(?:\\\[trn])+/

these patterns – to my surprise – both work. Why are these extra backslashes necessary?

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1 Answer

  1. Editorial Team

    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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