Possible match for (seemingly) invalid Lua-pattern

Question

I know that you can't repeat match groups in Lua. For example, if I wanted to match the two successive "45"'s, I can't do:

print(string.find("some 4545 text", "(%d%d)+"))

which will print nil (no match found).

However, since find(...) doesn't report an error (for the invalid patterns "%" and "(%d" errors are produced), it leads me to believe the pattern "(%d%d)+" is a valid one.

If "(%d%d)+" is a valid pattern, what does it match? And if it isn't, is there a particular reason no error is produced?

Answer 1

"(%d%d)+" is a valid pattern. It matches for example "some 45+67 text" or "some 4567+ text" and captures "45" in the first case and "67" in the second.

Answer 2

To match two successive occurrences of a string of digits, use "(%d+)%1".