Generated by SassMeister.com.

input.scss

/// Check if a string contains a number.
@function string-is-number($string) {
  // Matrices
  $numbers: '0' '1' '2' '3' '4' '5' '6' '7' '8' '9';

  @for $i from 1 through str-length($string) {
      $character: str-slice($string, $i, $i);
      $index: index($numbers, $character);
    
      @if $index {
        @return true;
      }
    }

  @return false;
}

/// Checks a string for the presence of a number.
/// If it contains a number, append a separator to it.
@function split-string($string, $separator) {
    @for $i from 1 through str-length($string) {
        $character: str-slice($string, $i, $i);
        $next-character: str-slice($string, $i + 1, $i + 1);
        $previous-character: if($i == 1, false, str-slice($string, $i - 1, $i - 1));
        
        @if string-is-number($character) and not string-is-number($next-character) {
            $string: str-insert($string, $separator, $i + 1);
        }
    }

    @return $string;
}


/// Testing
body {
    margin-top: split-string('20xl', '-');
}

output.css

body {
  margin-top: "20-xl";
}

settings.json

{
  "sass": {
    "compiler": "dart-sass/1.32.12",
    "extensions": {},
    "syntax": "SCSS",
    "outputStyle": "expanded"
  },
  "autoprefixer": false
}