Working with Notepad++ v7.9 here.
Suppose R is a complex regexp pattern. I now want to match _R, R_, or_R_ but not R alone. How to do this without writing explicitly (_R|R_|_R_)? This requires R being written out three times and looks ugly.
The closest I can think of is _?R_? but this also matches R alone (and is in fact equivalent to it), which is a false positive to me.
In between is (_?R_|_R_?) but R is repeated again here, though one less time.
You may use If-Then-Else as follows:
(_)?R(?(1)_?|_)
Demo.
This enables you to write R only once.
Breakdown:
(_)? # An optional capturing group that matches an underscore character.
R # Matches 'R' literally (replace it with your pattern).
(? # If...
(1) # ..the first capturing group is found,...
_? # ..then match zero or one underscore characters (optional).
| # Else (otherwise)...
_ # Match exactly one underscore character (required).
) # End If.
Works in Notepad++:
