Regular Expressions: "undefined group number 1"

Question

I am trying to do a relatively simple search+replace as follows: 
 find: 
 ([a-z])\.([A-Z]) 
 replace with: 
 \1. \2 
 
 (The intent is to make sure that if a target segment has two or more clauses separated by a period, that period is followed by a space.) 
 (The reason why this is necessary: bad alignment, resulting in target segments such as: 
 This is a claus e.T his is another one. 
 
 However, Studio (2017) does not recognize the groups in the "replace with" field, claiming those groups are undefined; see screenshot:

This syntax would work in the regexp applications I'm used to. How is it different in Studio?

Paul Filkin · Accepted Answer

Hi Florian Betz , 
 
Try $1 and $2 
 
Studio uses .Net for regex and \1 and \2 are normally used for back references in the search pattern, not in the replace.

Trados Studio > 5. Regex and XPath

Regular Expressions: "undefined group number 1"