42_         // invalid: _ must separate successive digits
4__2        // invalid: only one _ at a time
0_xBadFace  // invalid: _ must separate successive digits
'aa'         // illegal: too many characters
'\xa'        // illegal: too few hexadecimal digits
'\0'         // illegal: too few octal digits
'\uDFFF'     // illegal: surrogate half
'\U00110000' // illegal: invalid Unicode code point
"\uD800"             // illegal: surrogate half
"\U00110000"         // illegal: invalid Unicode code point