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s/\.\K[^.]*$/txt/;
Compiling REx "(?<=[.])[^.]*$" Final program: ... 7 <xyz.foo> <> | 19: EOL(20) 7 <xyz.foo> <> | 20: END(0) Match successful!
Compiling REx "\.\K[^.]*$" Final program: ... 7 <xyz.foo> <> | 16: EOL(17) 7 <xyz.foo> <> | 17: END(0) Match successful!