Question

Let me preface this by saying I’m a complete amateur when it comes to RegEx and only started a few days ago. I’m trying to solve a problem formatting a file and have hit a hitch with a particular type of data. The input file is structured like this:

 Two words,Word,Word,Word,'Number, number' 

What I need to do is format it like this…

 'Two words','Word',Word','Word','Number, number' 

I have had a RegEx pattern of

s/,/','/g

working, except it also replaces the comma in the already quoted Number, number section, which causes the field to separate and breaks the file. Essentially, I need to modify my pattern to replace a comma with ‘,’ [quote comma quote], but only when that comma isn’t followed by a space. Note that the other fields will never have a space following the comma, only the delimited number list.

I managed to write up

s/,[A-Za-z0-9]/','/g

which, while matching the appropriate strings, would replace the comma AND the following letter. I have heard of backreferences and think that might be what I need to use? My understanding was that

s/(,)[A-Za-z0-9]\b

should work, but it doesn’t.

Anyone have an idea?

Answer 1

s/,([^ ])/','$1/ will match a ‘,‘ followed by a ‘not-a-space’, capturing the not-a-space, then replacing the whole thing with the captured part.

Depending on which regex engine you’re using, you might be writing \1 or other things instead of $1.

If you’re using Perl or otherwise have access to a regex engine with negative lookahead, s/,(?! )/','/ (a ‘,‘ not followed by a space) works.

Your input looks like CSV, though, and if it actually is, you’d be better off parsing it with a real CSV parser rather than with regexes. There’s lot of other odd corner cases to worry about.