Force reddit to use the Legacy Search mode (by adding the feature=legacy_search URI parameter) to reddit searches.

reddit-legacy-search.user.js

// ==UserScript==
// @name         reddit Legacy Search
// @namespace    http://github.com/karlding
// @version      0.1
// @description  Force the Legacy Search mode by adding the `feature=legacy_search` URI parameter to reddit searches
// @author       Karl Ding
// @match        *://*.reddit.com/*
// @grant        none
// ==/UserScript==

/**
 * Uses the urlobject.js code from 
 *    https://gist.github.com/aymanfarhat/5608517
 */
function urlObject(options) {
    "use strict";
    /*global window, document*/

    var url_search_arr,
        option_key,
        i,
        urlObj,
        get_param,
        key,
        val,
        url_query,
        url_get_params = {},
        a = document.createElement('a'),
        default_options = {
            'url': window.location.href,
            'unescape': true,
            'convert_num': true
        };

    if (typeof options !== "object") {
        options = default_options;
    } else {
        for (option_key in default_options) {
            if (default_options.hasOwnProperty(option_key)) {
                if (options[option_key] === undefined) {
                    options[option_key] = default_options[option_key];
                }
            }
        }
    }

    a.href = options.url;
    url_query = a.search.substring(1);
    url_search_arr = url_query.split('&');

    if (url_search_arr[0].length > 1) {
        for (i = 0; i < url_search_arr.length; i += 1) {
            get_param = url_search_arr[i].split("=");

            if (options.unescape) {
                key = decodeURI(get_param[0]);
                val = decodeURI(get_param[1]);
            } else {
                key = get_param[0];
                val = get_param[1];
            }

            if (options.convert_num) {
                if (val.match(/^\d+$/)) {
                    val = parseInt(val, 10);
                } else if (val.match(/^\d+\.\d+$/)) {
                    val = parseFloat(val);
                }
            }

            if (url_get_params[key] === undefined) {
                url_get_params[key] = val;
            } else if (typeof url_get_params[key] === "string") {
                url_get_params[key] = [url_get_params[key], val];
            } else {
                url_get_params[key].push(val);
            }

            get_param = [];
        }
    }

    urlObj = {
        protocol: a.protocol,
        hostname: a.hostname,
        host: a.host,
        port: a.port,
        hash: a.hash.substr(1),
        pathname: a.pathname,
        search: a.search,
        parameters: url_get_params
    };

    return urlObj;
}


function buildUrl(url) {
    var href = '';
    
    // build URL
    href += url.protocol + '//';
    href += url.hostname;
    href += url.pathname;
    
    var params = '?';
    
    for (var key in url.parameters) {
        if (url.parameters.hasOwnProperty(key)) {
            params += key + "=" + url.parameters[key] + '&';
        }
    }
    params = params.slice(0, - 1); // remove trailing &
    
    href += params;
    return href;
}


// entry point
if((document.location.href).indexOf('search?q=') !== -1) {
    var url = urlObject();
    
    if(typeof url.parameters['feature'] === 'undefined') {
        url.parameters['feature'] = 'legacy_search';
        
        window.location = buildUrl(url);
    }
}