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<? add_filter( 'wpcf7_validate_textarea', 'cf7wl_textarea_validation_filter', 10, 2 );
add_filter( 'wpcf7_validate_textarea*', 'cf7wl_textarea_validation_filter', 10, 2 );

function cf7wl_textarea_validation_filter( $result, $tag ) {
  $tag = new WPCF7_Shortcode( $tag );
 $name = $tag['name'];

 if($name == 'your-message'){ 
    if (preg_match("/((http|https)\:\/\/)?[a-zA-Z0-9\.\/\?\:@\-_=#]+\.([a-zA-Z0-9\&\.\/\?\:@\-_=#])*/",$_POST[$name])) {
  $result->invalidate( $name, 'Сообщение не должно содержать ссылки' );
    }
 }
 return $result;
}