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Editorial Team
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Asked: 2026-05-22T18:28:08+00:00

I’ve encountered the following token in a regular expression: [\s\S]*? If I understand this

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I’ve encountered the following token in a regular expression: [\s\S]*?

If I understand this correctly, the character class means “match a whitespace character or a non-whitespace character”. Therefore, would this not do exactly the same thing as .*?

One possible difference is that usually . does not match newlines. However, this regular expression was written in Ruby and was passed the m modifier meaning that the . does, in fact, match newlines.

Is there any other reason to use [\s\S]*? instead of .*?

In case it helps, the regular expression I am looking at appears inside the sprockets library in the HEADER_PATTERN constant on line 97. The full expression is:

/
  \A \s* (
    (\/\* ([\s\S]*?) \*\/) |
    (\#\#\# ([\s\S]*?) \#\#\#) |
    (\/\/ ([^\n]*) \n?)+ |
    (\# ([^\n]*) \n?)+
  )
/mx
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1 Answer

  1. Editorial Team

    You interpreted the regex correctly.

    That seems like a relict from other languages which do not support the m-flag (or s-flag in other implementations).

    A reason to use that construct would be to not use the m-flag so you have the possibility to use . without matching newlines but are still able to match everything if need be.

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