Android WebView backspace functionality

Question

I am creating an android app that uses the MathQuill library. The application consists of a WebView that uses the MathQuill equation parser and a keybord (implemented via activity_main.xml and MainActivity.java) which has buttons that write math symbols to a WebView.

I need to create a backspace button functionality which I don't know how to do.

Backspace button in layout file:

<Button
    android:id="@+id/button_backspace"
    android:layout_width="35dp"
    android:layout_height="35dp"
    android:text="b"
    android:background="@drawable/button"
    android:padding="0dp"
    android:textSize="15sp"
    android:layout_margin="1dp"
/>

WebView in layout file:

<WebView
    android:id="@+id/web_view"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:descendantFocusability="blocksDescendants"
    android:focusable="false"
    android:focusableInTouchMode="false"
/>

WebView init in MainActivity.java

et = (WebView) findViewById(R.id.web_view);
et.getSettings().setJavaScriptEnabled(true);
et.loadUrl("file:///android_asset/some.html");

Backspace button in MainActivity.java:

Button btn_backspace = (Button) findViewById(R.id.button_backspace);
btn_backspace.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        et.loadUrl("javascript:input(\"\\b\")");
    }
});

Android asset html fragment:

var MQ = MathQuill.getInterface(2); 
var mathField = MQ.MathField(mathFieldSpan, {
  spaceBehavesLikeTab: true,
  handlers: {
    edit: function() {
      mathField.focus()
      //latexSpan.textContent = mathField.textContent
      latexSpan.textContent = mathField.latex();
      Android.getImageUrl(mathField.latex())
    }
  }
});

function input(str) {
    if(str == "tangens"){
        str = "\\tan"
    }
    mathField.cmd(str)
    mathField.focus()
}

the input function is used to write math symbols to a WebView.

I tried sending a \b char with no success.