presto-main/src/main/java/io/prestosql/operator/TypeSignatureParser.java/TypeSignatureParser/isValidStartOfIdentifier
class TypeSignatureParser:    private static boolean isValidStartOfIdentifier(char c)
    {
        return (c >= 'a' && c <= 'z') ||
                (c >= 'A' && c <= 'Z') ||
                c == '_';
    }