This doesn't seem difficult, but the instructions are very unclear. Skipping due to lack of clear instructions.

(?m) multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)

Didn't know you could edit the test code. Yep, you're right.

Would you happen to have any hints to why my code is wrong?

Is that a sample test? Then maybe you edited it, and no, it should be Runes.solveExpression.

One of the tests (which returns an error) says:

"assertEquals( "Answer for expression '?*11=??' " , 2 , Runes.sol_____________veExpression("?*11=??") );"

Is that giant space supposed to be there?

Apart from that, I am receiving errors but unable to think of test cases that invalidate my code.

Needs better description and invalidates correct answers (I succeeded after rewriting code with same return output), bad kata