If you've ever needed to collect Twitter or Instagram handles, then this input mask allows you to mask a field so that the @ symbol prepends the username value being provided.

// Provides an account handle input mask to receive Twitter and Instagram account handles
add_filter( 'gform_input_masks', function( $masks ) {

    $masks['Account Handle'] = '@*?**************************';
    return $masks;

} );

add_filter( 'gform_input_mask_script', function( $script, $form_id, $field_id, $mask ) {

    if ( $mask == '@*?**************************' ) {
        $script = "jQuery.mask.definitions['*']='[A-Z,a-z,0-9,_]';
                   jQuery('#input_{$form_id}_{$field_id}')
                       .mask('".esc_js( $mask )."',{placeholder:' '})
                       .on('keypress',function(e){if(e.which==13){jQuery(this).blur();}});";
    }
    return $script;

}, 10, 4 );