Question

This was a fascinating debugging experience.
Can you spot the difference between the following two lines?

StringReplace["–", RegularExpression@"[\\s\\S]" -> "abc"]
StringReplace["-", RegularExpression@"[\\s\\S]" -> "abc"]

They do very different things when you evaluate them. It turns out it’s because the string being replaced in the first line consists of a unicode en dash, as opposed to a plain old ascii dash in the second line.

In the case of the unicode string, the regular expression doesn’t match.
I meant the regex “[\s\S]” to mean “match any character (including newline)” but Mathematica apparently treats it as “match any ascii character”.

How can I fix the regular expression so the first line above evaluates the same as the second? Alternatively, is there an asciify filter I can apply to the strings first?

PS: The Mathematica documentation says that its string pattern matching is built on top of the Perl-Compatible Regular Expressions library (http://pcre.org) so the problem I’m having may not be specific to Mathematica.

Answer 1

Here’s an asciify function which I used as a workaround at first:

f[s_String] := s
f[x_] := FromCharacterCode[x]

asciify[s_String] := 
  StringJoin[f /@ (ToCharacterCode[s] /. x_?(#>255&) :> "&"<>ToString[x]<>";")]

Then I realized, thanks to @Isaac’s answer, that “.” as a regular expression doesn’t seem to have this unicode problem. I learned from the answers to Bug in Mathematica: regular expression applied to very long string that “(.|\n)” is ill-advised but that “(?s).” is recommended. So I think the best fix is the following:

StringReplace["–", RegularExpression@"(?s)." -> "abc"]