JSFiddle - React, Tailwind, and code Playground

HTML

<div id="result"></div>

JavaScript

var title = "this is simple string with finnish word tämä on ääkköstesti älkää ihmetelkö";
// Does not work
var searchterm = "äl";

// does not work
//var searchterm = "ää";

// Works
//var searchterm = "wi"; 
    //alert(asciiOnly(title));
    //alert(asciiOnly(searchterm));
if ( new RegExp("\\b"+asciiOnly(searchterm), "gi").test(asciiOnly(title)) ) {
    $("#result").html("Match: ("+searchterm+"): "+title);
} else {
    $("#result").html("nothing found with term: "+searchterm);   
}

    function asciiOnly(input) {
      var output = "";
for (var i = 0; i < input.length; i++) {

    if (input.charCodeAt(i) < 128) {
      output += input.charAt(i);            
    }
    else {
        percents = encodeURI(input.charAt(i));
        percents = 'QQ' + percents;
        output += percents.replace(/%/g, "");
    }
}
          return output;
    
    }