I am working with a document containing thousands of serial numbers. I want to insert a hyphen i the middle of each number using "find and replace"

The challenge is to replace all serial numbers in the document that look like this: Mxxxxxxxxxx with this Mxxxx-xxxxxx

Using find/replace with jokers, i am able to find all serial numbers that i want to change. M^9^9^9^9 but when i want to replace them and insert a – by typing this, M^9^9^9^9–, the old numbers disappear.

It results in the following example:
M6015803710 ––> M^9^9^9^9-803710
Instead of:
M6015803710 ––> M6015-803710

Thanks!