StrCpy $R0 0
StrCpy $R4 ""
StrLen $R5 $YourVar
${Do}
  ${If} $R0 = $R5
    ${Break}
  ${EndIf}
  ;get char
  StrCpy $R1 $YourVar 1 $R0
  ;get ascii-number of char
  System::Call "*(&t1 r11)i.r12"
  System::Call "*$R2(&i1 .r13)"
  System::Free $R2
  ${If} $R3 < 65
  ${OrIf} $R3 > 90
    ;if non-alpha keep this char
    StrCpy $R4 $R4$R1
  ${EndIf}
  IntOp $R0 $R0 + 1
${Loop}