String Replacement function in pure-sass

str-replace.scss

@function str-replace($source, $substring, $replacement, $fail-silently: false) {
  $start: str-index($source, $substring);
  @if $start {
    @return str-slice($source, 1, $start - 1)  + $replacement + str-slice($source, $start + str-length($substring))
  }
  @if $fail-silently {
    @return $source;
  } @else {
    @error "'#{$substring}' was not found in '#{$source}'";
  }
}

@debug str-replace("beforeDURINGafter", "DURING", "REPLACED");