This warning was introduced in 3.6. The reason is that in most other regular expression implementations global inline flags in the middle of the expression have different semantic: they affect only the part of the expression after the flag. But in Python they affect the whole expression. It caused confusion and was a source of bugs.

After 5 releases it is a time to convert this warning into error. In future we can allow global inline flags in the middle of the expression with different semantic. It is safer if one or more intermediate versions will raise an error.