Padding a string in cmake

pad.cmake

function(pad_string output str padchar length)
  string(LENGTH "${str}" _strlen)
  math(EXPR _strlen "${length} - ${_strlen}")

  if(_strlen GREATER 0)
    if(${CMAKE_VERSION} VERSION_LESS "3.14")
      unset(_pad)
      foreach(_i RANGE 1 ${_strlen}) # inclusive
        string(APPEND _pad ${padchar})
      endforeach()
    else()
      string(REPEAT ${padchar} ${_strlen} _pad)
    endif()
    string(APPEND str ${_pad})
  endif()

  set(${output} "${str}" PARENT_SCOPE)
endfunction()

A not so generic, but much shorter solution:

string(REGEX REPLACE "^.*(....)\$" "\\1" unicode "0000${unicode}")

also thanks @jtanx!!!