Googles Apps Script exact match

I'm using this script but it's replacing every instance of a T, A, etc. How do I get it to only replace an exact match? Only if it's the letter T and nothing else.

function runReplaceInSheet(){
  var sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName("Underlevel");
  //  get the current data range values as an array
  //  Fewer calls to access the sheet -> lower overhead 
  var values = sheet.getDataRange().getValues();  

  // Replace
  replaceInSheet(values, "/^T$/", '=image("https://i.imgur.com/Dxl893F.png")');
  replaceInSheet(values, 'A', '=image("https://i.imgur.com/omc7F9l.png")');
  replaceInSheet(values, 'R', '=image("https://i.imgur.com/12ZmSp3.png")');
  replaceInSheet(values, 'M', '=image("https://i.imgur.com/kh7RqBD.png")');
  replaceInSheet(values, 'H', '=image("https://i.imgur.com/u0O7fsS.png")');
  replaceInSheet(values, 'F', '=image("https://i.imgur.com/Hbs3TuP.png")');

  // Write all updated values to the sheet, at once
  sheet.getDataRange().setValues(values);
}

function replaceInSheet(values, to_replace, replace_with) {
  //loop over the rows in the array
  for(var row in values){
    //use Array.map to execute a replace call on each of the cells in the row.
    var replaced_values = values[row].map(function(original_value) {
      return original_value.toString().replace(to_replace,replace_with);
    });

    //replace the original row values with the replaced values
    values[row] = replaced_values;
  }
}

Thank you :D