I have been working with the regular expression package (Text.Regex.Posix). My hope was to find a simple way to remove a pair of XML tags from a short string.

I have something like this "<tag>Data</tag>" and would like to extract 'Data'. There is only one tag pair, no nesting, and I know exactly what the tag is.

My first attempt was this:

"<tag>123</tag>" =~ "[^<tag>].+[^</tag>]"::String

result: "123"

Upon further experimenting I realized that it only works with more than 2 digits in 'Data'. I occured to me that my thinking on how this regular expression works was not correct - but I don't understand why it works at all for 3 or more digits.

Can anyone help me understand this result and perhaps suggest another strategy? Thank you.

The regex you are using here can be described as such:

"Match a character not in the set '<,t,a,g,>', followed by 1 or more of anything, followed by a character not in the set '<,/,t,a,g,>'."

Effectively, it will not match if your data has less than 3 characters and is probably not the correct regex for this job, i.e. it would also match "x123x". What you need is regex capturing, but I don't know if that is available in that regex library (I'm not an expert Haskeller).

If you really need a regex to locate the tag, you could use a function like this to extract it:

getTagData tag s =
   let match = s =~ ("<" ++ tag ++ ">.*</" ++ tag ++ ">")::String
       dropTag = drop (length tag + 2) s
       getData = take (length match - (2 * length tag + 5)) dropTag
in if length match > 0
       then Just getData
       else Nothing

*Main> getTagData "tag" "<tag>123</tag>"
Just "123"

Patrick

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