The task is pretty simple, but I've not been able to come up with a good solution yet: a string can contain numbers, dashes and pluses, or only numbers.
does most of what I need, except when a user enters garbage like "+-+–+"
I've not had luck with regular lookahead, since the dashes and pluses could potentially be anywhere in the string.
How about this:
which means "one or more digits, each of which can be preceded or followed by an optional plus or minus".
Here's a Python test script:
which prints this: