Regular expression, search. How to properly implement negation?

Question

Regular Expressions

There is a regular expression - ^[A-z0-9]*[\_][A-z0-9].*$
It looks for lines that contain "_". But the trouble is that the separator in the line can occur several times.
Need: select the lines in which it occurs only ONCE . I don’t really understand regular expressions, but I’m learning.

Answer 1

^[a-zA-Z0-9]*_[a-zA-Z0-9]*[^_]*$