It looks like you're using the regex module to append your text, and you can (but use $1 instead of \1 - don't ask me why, that a Y! pipes peculiarity).
However, the regex module has limits on the length of the input string, and using a regex operation for a simple addition is an overkill.
Use a string builder instead, inside a loop module. Click on the + for a second field, write whatever you want in the first input field and the name of the field to append the text to in the second field.