Looking at ASCII values, '.' (ASCII 46) comes before 'm' (ASCII 109). This works for comparing basic punctuation to letters, however you should remember that in ASCII capital 'Z' comes before lowercase 'a'.
UTS #10: Unicode Collation Algorithm covers comparing strings that contain punctuation.
This report is the specification of the Unicode Collation Algorithm (UCA), which details how to compare two Unicode strings while remaining conformant to the requirements of the Unicode Standard. The UCA also supplies the Default Unicode Collation Element Table (DUCET) as the data specifying the default collation order for all Unicode characters.