I have a very large TeX-File wherein we set mathematical text. Unfortunately, TeX sets a empty space after each comma, even though numbers (in Germany) are formatted without space after comma.

I am aware that there are packages that solve this problem automatically, but I tried to find such commas with a regular expression before the TeX formatter would do its thing, so I could wrap these commas in braces. For example, I want 1,2 to become 1{,}2. But my regular expression approach does not work as expected:

I used "[0-9],[0-9]" resp. "\d,\d" to look for all places where two digits are separated with a decimal comma. That works fine. But I was wondering why for input "1,2,3" it only finds the match "1,2" but not "2,3".

There must be something I don’t quite understand about regular expressions. How would I have to alter the expression to find overlapping instances of the search pattern?

>Solution :

Once a character has been matched, it is not matched again. As in your example "2" is matched, the "cursor" of the regex engine has now passed that "2", and so the next match can only be found in ",3" — and there no match is found.

To resolve that, use a look-ahead assertion for the decimal digits without actually capturing them:

\d,(?=\d)

The matches will be "1," and "2," and now you can make the replacement as you want (adding braces), knowing these matches guarantee there is a digit following.