The Columns++ approach is a good one, and if your file isn’t several megabytes or larger, I would just stick with that and ignore the rest of this post.
That said, you could get better performance on very large files using PythonScript, with the following script:
import re
x = [0]
def on_match(m):
x[0] += 1
return m.group(1) + str(x[0])
editor.setText(re.sub('(abc_)1', on_match, editor.getText()))
Note that there’s a superficially similar script you could write with PythonScript that uses editor.rereplace instead of re.sub, but editor.rereplace will be much slower than re.sub for very large files.