Regex matches tags like <a>, <bb>, <ccc>, but not <abc>

Question

I need a regex to match tags that look like <A> , <BB> , <CCC> but not <ABC> , <aaa> , <> . therefore, the tag must consist of the same capital letter, repeated. I tried <[AZ]+> but this does not work. of course, I can write something like <(A+|B+|C+|...)> and so on, but I wonder if there is a more elegant solution.

Answer 1

You can use something like this ( see this at rubular.com ):

 <([AZ])\1*> 

In this case, the capture group and backlink are used. Mostly:

• You use (pattern) to "capture" a match
• Then you can use \n in your template, where n is the group number to “link back” to what matches that group.

So in this case:

• Group 1 captures ([AZ]) , an uppercase letter immediately after <
• Then we see if we can match \1* , i.e. zero or more of the same letters

References

• regular-expressions.info/Grouping and Backreference