renaming variable names inside regular expressions does not work

[zenogantner]

How to reproduce:

• load the file Padre/Document/Java/FunctionList.pm from the Padre source (it exists in r16930)
• move cursor to the line where the variable $n is defined
• use Alt-Shift-R to rename (works)
• now do the same with the occurrence of this variable inside the regular expression below
• result: You get a prompt for renaming, but an error message afterwards