Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.4.1)
// Compass (v1.0.1)
// ----

@function isNaN($val) { @return type-of($val)!=number; }



@function convert-to-unit($string){
  @if(not isNaN($string) ){ @return $string; }            // It is a valid unit/number 
  $string:to-lower-case($string);                         // Convert to lowercase 
  $numbers:'0','1','2','3','4','5','6','7','8','9';       // Numbers
  $units:(px:1px, pt:1pt, pc:1pc, in:1in, mm:1mm, cm:1cm, // Absolute
          em:1em, rem:1rem, ch:1ch, ex:1ex,               // Relative - font based
          vv:1vw, vh:1vh, vmin:1vmin, vmax:1vmax,         // Relative - viewport based
          deg:1deg, turn:1turn, grad:1grad,               // Angles
          s:1s, ms:1ms,                                   // Time
          hz:1hz, kHz:1khz,                               // Frequencies
          dpi:1dpi, dpcm:1dpcm, dppx:1dppx,                // Resolutions
  );
  
  // Variables
  $number:0;
  $devider:0;
  $digits:false; // change to counter to catch multi dots
  $unit:'';
  
  @for $i from 1 through str-length($string) {
    $char:str-slice($string, $i, $i);
    $index:index($numbers,$char);
    $digits:if($char=='.', true,  $digits);
    @if($index){
      $devider:if($digits ,$devider + 1, $devider);
      $number: $number * 10 + $index - 1; 
    }
    @if(not $index and not index('-' '.',$char)){
      $unit:$unit + $char;
    }
  }  
  // Negative
  $number:if(str-slice($string, 1, 1)=='-',$number * -1,$number);
  // Decimals
  $number:$number / pow(10, $devider);
  
  // No unit found return number
  @if($unit == ''){ @return $number; }
  // Unit found check if valid 
  $unit:map-get($units,$unit);
  @if(not $unit){ @warn "Unknown unit `#{$unit}`."; @return null; } // <-- change "error" to null
  @return $unit * $number; 
}


.foo { hey:convert-to-unit('-123.7243--R.E.M'); isNaN:isNaN(-123.7243turn);}

@function number($val){ @return convert-to-unit($val)}
sass {
  cast: convert-to-unit("-15");    // -15
  cast: convert-to-unit("-1");     // -1
  cast: convert-to-unit("-.5");    // -.5
  cast: convert-to-unit("-0");     // 0
  cast: convert-to-unit("0");      // 0
  case: convert-to-unit(".10");    // 0.1
  cast: convert-to-unit("1");      // 1
  cast: convert-to-unit("1.5");    // 1.5
  cast: convert-to-unit("10.");    // 10
  cast: convert-to-unit("12.380"); // 12.38
  cast: convert-to-unit("42");     // 42
  cast: convert-to-unit("1337");   // 1337
  
  cast: convert-to-unit("-10px");  // -10px
  cast: convert-to-unit("20em");   // 20em
  cast: convert-to-unit("30ch");   // 30ch
  
  cast: convert-to-unit("1fail");  // Error
  cast: convert-to-unit("string"); // Error
}

SassMeister-output.css

.foo {
  hey: -123.7243rem;
  isNaN: false;
}

sass {
  cast: -15;
  cast: -1;
  cast: -0.5;
  cast: 0;
  cast: 0;
  case: 0.1;
  cast: 1;
  cast: 1.5;
  cast: 10;
  cast: 12.38;
  cast: 42;
  cast: 1337;
  cast: -10px;
  cast: 20em;
  cast: 30ch;
}