nl_tokens: ES#sec - advancestringindex AdvanceStringIndex ( S index unicode )
pl_tokens: function AdvanceStringIndex ( string , index , unicode ) { var increment = 1 ; if ( unicode ) { var first = % _StringCharCodeAt ( string , index ) ; if ( first >= 0xD800 && first <= 0xDBFF && string . length > <mask> + 1 ) { var second = % _StringCharCodeAt ( string , index + 1 ) ; if ( second >= 0xDC00 && second <= 0xDFFF ) { increment = 2 ; } } } return increment ; }