diff --git a/frontend_tests/node_tests/compose.js b/frontend_tests/node_tests/compose.js
index 760650540d..f310cbb2a4 100644
--- a/frontend_tests/node_tests/compose.js
+++ b/frontend_tests/node_tests/compose.js
@@ -1487,7 +1487,7 @@ run_test("on_events", () => {
         handler(ev);
 
         // video link ids consist of 15 random digits
-        let video_link_regex = /\[translated: Click to join video call\]\(https:\/\/meet.jit.si\/\d{15}\)/;
+        let video_link_regex = /\[translated: Click to join video call]\(https:\/\/meet.jit.si\/\d{15}\)/;
         assert.match(syntax_to_insert, video_link_regex);
 
         page_params.jitsi_server_url = null;
@@ -1512,7 +1512,7 @@ run_test("on_events", () => {
         };
 
         handler(ev);
-        video_link_regex = /\[translated: Click to join video call\]\(example\.zoom\.com\)/;
+        video_link_regex = /\[translated: Click to join video call]\(example\.zoom\.com\)/;
         assert.match(syntax_to_insert, video_link_regex);
 
         page_params.realm_video_chat_provider =
@@ -1527,7 +1527,7 @@ run_test("on_events", () => {
         };
 
         handler(ev);
-        video_link_regex = /\[translated: Click to join video call\]\(\/calls\/bigbluebutton\/join\?meeting_id=%22zulip-1%22&password=%22AAAAAAAAAA%22&checksum=%2232702220bff2a22a44aee72e96cfdb4c4091752e%22\)/;
+        video_link_regex = /\[translated: Click to join video call]\(\/calls\/bigbluebutton\/join\?meeting_id=%22zulip-1%22&password=%22AAAAAAAAAA%22&checksum=%2232702220bff2a22a44aee72e96cfdb4c4091752e%22\)/;
         assert.match(syntax_to_insert, video_link_regex);
     })();
 
diff --git a/frontend_tests/node_tests/markdown.js b/frontend_tests/node_tests/markdown.js
index ce8b1f4bd4..c815311b3c 100644
--- a/frontend_tests/node_tests/markdown.js
+++ b/frontend_tests/node_tests/markdown.js
@@ -676,19 +676,19 @@ run_test("python_to_js_filter", () => {
     // to update_realm_filter_rules.
     markdown.update_realm_filter_rules([["/a(?im)a/g"], ["/a(?L)a/g"]]);
     let actual_value = marked.InlineLexer.rules.zulip.realm_filters;
-    let expected_value = [/\/aa\/g(?![\w])/gim, /\/aa\/g(?![\w])/g];
+    let expected_value = [/\/aa\/g(?!\w)/gim, /\/aa\/g(?!\w)/g];
     assert.deepEqual(actual_value, expected_value);
     // Test case with multiple replacements.
     markdown.update_realm_filter_rules([
-        ["#cf(?P<contest>[0-9]+)(?P<problem>[A-Z][0-9A-Z]*)", "http://google.com"],
+        ["#cf(?P<contest>\\d+)(?P<problem>[A-Z][\\dA-Z]*)", "http://google.com"],
     ]);
     actual_value = marked.InlineLexer.rules.zulip.realm_filters;
-    expected_value = [/#cf([0-9]+)([A-Z][0-9A-Z]*)(?![\w])/g];
+    expected_value = [/#cf(\d+)([A-Z][\dA-Z]*)(?!\w)/g];
     assert.deepEqual(actual_value, expected_value);
     // Test incorrect syntax.
     blueslip.expect(
         "error",
-        "python_to_js_filter: Invalid regular expression: /!@#@(!#&((!&(@#((?![\\w])/: Unterminated group",
+        "python_to_js_filter: Invalid regular expression: /!@#@(!#&((!&(@#((?!\\w)/: Unterminated group",
     );
     markdown.update_realm_filter_rules([["!@#@(!#&((!&(@#(", "http://google.com"]]);
     actual_value = marked.InlineLexer.rules.zulip.realm_filters;
diff --git a/frontend_tests/puppeteer_tests/16-settings.js b/frontend_tests/puppeteer_tests/16-settings.js
index bcf5fe895e..b7a03c93be 100644
--- a/frontend_tests/puppeteer_tests/16-settings.js
+++ b/frontend_tests/puppeteer_tests/16-settings.js
@@ -8,7 +8,7 @@ const common = require("../puppeteer_lib/common");
 const OUTGOING_WEBHOOK_BOT_TYPE = "3";
 const GENERIC_BOT_TYPE = "1";
 
-const zuliprc_regex = /^data:application\/octet-stream;charset=utf-8,\[api\]\nemail=.+\nkey=.+\nsite=.+\n$/;
+const zuliprc_regex = /^data:application\/octet-stream;charset=utf-8,\[api]\nemail=.+\nkey=.+\nsite=.+\n$/;
 
 async function get_decoded_url_in_selector(page, selector) {
     return await page.evaluate(
@@ -76,7 +76,7 @@ async function test_get_api_key(page) {
 
     await page.waitForSelector("#show_api_key", {visible: true});
     const api_key = await common.get_text_from_selector(page, "#api_key_value");
-    assert(/[a-zA-Z0-9]{32}/.test(api_key), "Incorrect API key format.");
+    assert(/[\dA-Za-z]{32}/.test(api_key), "Incorrect API key format.");
 
     const download_zuliprc_selector = "#download_zuliprc";
     await page.click(download_zuliprc_selector);
@@ -97,7 +97,7 @@ async function test_webhook_bot_creation(page) {
 
     const bot_email = "1-bot@zulip.testserver";
     const download_zuliprc_selector = '.download_bot_zuliprc[data-email="' + bot_email + '"]';
-    const outgoing_webhook_zuliprc_regex = /^data:application\/octet-stream;charset=utf-8,\[api\]\nemail=.+\nkey=.+\nsite=.+\ntoken=.+\n$/;
+    const outgoing_webhook_zuliprc_regex = /^data:application\/octet-stream;charset=utf-8,\[api]\nemail=.+\nkey=.+\nsite=.+\ntoken=.+\n$/;
 
     await page.waitForSelector(download_zuliprc_selector, {visible: true});
     await page.click(download_zuliprc_selector);
@@ -137,7 +137,7 @@ async function test_botserverrc(page) {
         page,
         "#download_botserverrc",
     );
-    const botserverrc_regex = /^data:application\/octet-stream;charset=utf-8,\[\]\nemail=.+\nkey=.+\nsite=.+\ntoken=.+\n$/;
+    const botserverrc_regex = /^data:application\/octet-stream;charset=utf-8,\[]\nemail=.+\nkey=.+\nsite=.+\ntoken=.+\n$/;
     assert(botserverrc_regex.test(botserverrc_decoded_url), "Incorrect botserverrc format.");
 }
 
diff --git a/static/js/buddy_data.js b/static/js/buddy_data.js
index ba15d8781f..eeefdc92f6 100644
--- a/static/js/buddy_data.js
+++ b/static/js/buddy_data.js
@@ -121,7 +121,7 @@ function filter_user_ids(user_filter_text, user_ids) {
 
     user_ids = user_ids.filter((user_id) => !people.is_my_user_id(user_id));
 
-    let search_terms = user_filter_text.toLowerCase().split(/[|,]+/);
+    let search_terms = user_filter_text.toLowerCase().split(/[,|]+/);
     search_terms = search_terms.map((s) => s.trim());
 
     const persons = user_ids.map((user_id) => people.get_by_user_id(user_id));
diff --git a/static/js/common.js b/static/js/common.js
index 0e36c70d91..b6593d1077 100644
--- a/static/js/common.js
+++ b/static/js/common.js
@@ -100,7 +100,7 @@ exports.copy_data_attribute_value = function (elem, key) {
 };
 
 exports.has_mac_keyboard = function () {
-    return /Mac/i.test(navigator.platform);
+    return /mac/i.test(navigator.platform);
 };
 
 exports.adjust_mac_shortcuts = function (key_elem_class, require_cmd_style) {
diff --git a/static/js/composebox_typeahead.js b/static/js/composebox_typeahead.js
index 33b7ca14c9..23cd5446aa 100644
--- a/static/js/composebox_typeahead.js
+++ b/static/js/composebox_typeahead.js
@@ -278,7 +278,7 @@ exports.tokenize_compose_str = function (s) {
             case "_":
                 if (i === 0) {
                     return s;
-                } else if (/[\s(){}[\]]/.test(s[i - 1])) {
+                } else if (/[\s()[\]{}]/.test(s[i - 1])) {
                     return s.slice(i);
                 }
                 break;
@@ -612,7 +612,7 @@ exports.get_candidates = function (query) {
         // as :P or :-p
         // Also, if the user has only typed a colon and nothing after,
         // no need to match yet.
-        if (/^:-.?$/.test(current_token) || /^:[^a-z+]?$/.test(current_token)) {
+        if (/^:-.?$/.test(current_token) || /^:[^+a-z]?$/.test(current_token)) {
             return false;
         }
         // Don't autocomplete if there is a space following a ':'
diff --git a/static/js/copy_and_paste.js b/static/js/copy_and_paste.js
index 9557ac8d76..649ee2d1d3 100644
--- a/static/js/copy_and_paste.js
+++ b/static/js/copy_and_paste.js
@@ -317,7 +317,7 @@ exports.paste_handler = function (event) {
         const paste_html = clipboardData.getData("text/html");
         if (paste_html && page_params.development_environment) {
             const text = exports.paste_handler_converter(paste_html);
-            const mdImageRegex = /^!\[.*\]\(.*\)$/;
+            const mdImageRegex = /^!\[.*]\(.*\)$/;
             if (text.match(mdImageRegex)) {
                 // This block catches cases where we are pasting an
                 // image into Zulip, which is handled by upload.js.
diff --git a/static/js/markdown.js b/static/js/markdown.js
index 0a78eef99d..ea0bcdef8c 100644
--- a/static/js/markdown.js
+++ b/static/js/markdown.js
@@ -30,12 +30,12 @@ const backend_only_markdown_re = [
     // Inline image previews, check for contiguous chars ending in image suffix
     // To keep the below regexes simple, split them out for the end-of-message case
 
-    /[^\s]*(?:(?:\.bmp|\.gif|\.jpg|\.jpeg|\.png|\.webp)\)?)\s+/m,
-    /[^\s]*(?:(?:\.bmp|\.gif|\.jpg|\.jpeg|\.png|\.webp)\)?)$/m,
+    /\S*(?:\.bmp|\.gif|\.jpg|\.jpeg|\.png|\.webp)\)?\s+/m,
+    /\S*(?:\.bmp|\.gif|\.jpg|\.jpeg|\.png|\.webp)\)?$/m,
 
     // Twitter and youtube links are given previews
 
-    /[^\s]*(?:twitter|youtube).com\/[^\s]*/,
+    /\S*(?:twitter|youtube).com\/\S*/,
 ];
 
 exports.translate_emoticons_to_names = (text) => {
@@ -88,7 +88,7 @@ exports.contains_backend_only_syntax = function (content) {
     // then don't render it locally. It is workaround for the fact that
     // javascript regex doesn't support lookbehind.
     const false_filter_match = realm_filter_list.find((re) => {
-        const pattern = /(?:[^\s'"(,:<])/.source + re[0].source + /(?![\w])/.source;
+        const pattern = /[^\s"'(,:<]/.source + re[0].source + /(?!\w)/.source;
         const regex = new RegExp(pattern);
         return regex.test(content);
     });
@@ -237,7 +237,7 @@ exports.add_topic_links = function (message) {
     }
 
     // Also make raw urls navigable
-    const url_re = /\b(https?:\/\/[^\s<]+[^<.,:;"')\]\s])/g; // Slightly modified from third/marked.js
+    const url_re = /\b(https?:\/\/[^\s<]+[^\s"'),.:;<\]])/g; // Slightly modified from third/marked.js
     const match = topic.match(url_re);
     if (match) {
         links = links.concat(match);
@@ -432,7 +432,7 @@ function python_to_js_filter(pattern, url) {
     }
     // Convert any python in-regex flags to RegExp flags
     let js_flags = "g";
-    const inline_flag_re = /\(\?([iLmsux]+)\)/;
+    const inline_flag_re = /\(\?([Limsux]+)\)/;
     match = inline_flag_re.exec(pattern);
 
     // JS regexes only support i (case insensitivity) and m (multiline)
@@ -456,7 +456,7 @@ function python_to_js_filter(pattern, url) {
     // is rendered locally, otherwise, we return false there and
     // message is rendered on the backend which has proper support
     // for negative lookbehind.
-    pattern = pattern + /(?![\w])/.source;
+    pattern = pattern + /(?!\w)/.source;
     let final_regex = null;
     try {
         final_regex = new RegExp(pattern, js_flags);
@@ -537,7 +537,7 @@ exports.initialize = function (realm_filters, helper_config) {
     disable_markdown_regex(marked.Lexer.rules.tables, "lheading");
 
     // Disable __strong__ (keeping **strong**)
-    marked.InlineLexer.rules.zulip.strong = /^\*\*([\s\S]+?)\*\*(?!\*)/;
+    marked.InlineLexer.rules.zulip.strong = /^\*\*([\S\s]+?)\*\*(?!\*)/;
 
     // Make sure <del> syntax matches the backend processor
     marked.InlineLexer.rules.zulip.del = /^(?!<~)~~([^~]+)~~(?!~)/;
@@ -545,7 +545,7 @@ exports.initialize = function (realm_filters, helper_config) {
     // Disable _emphasis_ (keeping *emphasis*)
     // Text inside ** must start and end with a word character
     // to prevent mis-parsing things like "char **x = (char **)y"
-    marked.InlineLexer.rules.zulip.em = /^\*(?!\s+)((?:\*\*|[\s\S])+?)((?:[\S]))\*(?!\*)/;
+    marked.InlineLexer.rules.zulip.em = /^\*(?!\s+)((?:\*\*|[\S\s])+?)(\S)\*(?!\*)/;
 
     // Disable autolink as (a) it is not used in our backend and (b) it interferes with @mentions
     disable_markdown_regex(marked.InlineLexer.rules.zulip, "autolink");
diff --git a/static/js/portico/tabbed-instructions.js b/static/js/portico/tabbed-instructions.js
index de6a003dc4..4649a27261 100644
--- a/static/js/portico/tabbed-instructions.js
+++ b/static/js/portico/tabbed-instructions.js
@@ -1,17 +1,17 @@
 export function detect_user_os() {
-    if (/Android/i.test(navigator.userAgent)) {
+    if (/android/i.test(navigator.userAgent)) {
         return "android";
     }
-    if (/iPhone|iPad|iPod/i.test(navigator.userAgent)) {
+    if (/iphone|ipad|ipod/i.test(navigator.userAgent)) {
         return "ios";
     }
     if (common.has_mac_keyboard()) {
         return "mac";
     }
-    if (/Win/i.test(navigator.userAgent)) {
+    if (/win/i.test(navigator.userAgent)) {
         return "windows";
     }
-    if (/Linux/i.test(navigator.userAgent)) {
+    if (/linux/i.test(navigator.userAgent)) {
         return "linux";
     }
     return "mac"; // if unable to determine OS return Mac by default
diff --git a/static/js/settings_bots.js b/static/js/settings_bots.js
index 6c4850ea0e..d3a494e224 100644
--- a/static/js/settings_bots.js
+++ b/static/js/settings_bots.js
@@ -68,7 +68,7 @@ function is_local_part(value, element) {
     // Adapted from Django's EmailValidator
     return (
         this.optional(element) ||
-        /^[-!#$%&'*+/=?^_`{}|~0-9A-Z]+(\.[-!#$%&'*+/=?^_`{}|~0-9A-Z]+)*$/i.test(value)
+        /^[\w!#$%&'*+/=?^`{|}~-]+(\.[\w!#$%&'*+/=?^`{|}~-]+)*$/i.test(value)
     );
 }
 
diff --git a/static/js/stream_color.js b/static/js/stream_color.js
index 01f04d5fe5..932ff1d1e7 100644
--- a/static/js/stream_color.js
+++ b/static/js/stream_color.js
@@ -154,13 +154,13 @@ exports.get_color_class = _.memoize((color) => {
     const channel = [0, 0, 0];
     let mult = 1;
 
-    match = /^#([\da-fA-F]{2})([\da-fA-F]{2})([\da-fA-F]{2})$/.exec(color);
+    match = /^#([\dA-Fa-f]{2})([\dA-Fa-f]{2})([\dA-Fa-f]{2})$/.exec(color);
     if (!match) {
         // 3-digit shorthand; Spectrum gives this e.g. for pure black.
         // Multiply each digit by 16+1.
         mult = 17;
 
-        match = /^#([\da-fA-F])([\da-fA-F])([\da-fA-F])$/.exec(color);
+        match = /^#([\dA-Fa-f])([\dA-Fa-f])([\dA-Fa-f])$/.exec(color);
         if (!match) {
             // Can't understand color.
             return "";
diff --git a/static/js/stream_data.js b/static/js/stream_data.js
index eaa7d33ddc..cfa07a94f7 100644
--- a/static/js/stream_data.js
+++ b/static/js/stream_data.js
@@ -315,7 +315,7 @@ exports.slug_to_name = function (slug) {
     GitHub conversations.  We migrated to modern slugs in
     early 2018.
     */
-    const m = /^([\d]+)(-.*)?/.exec(slug);
+    const m = /^(\d+)(-.*)?/.exec(slug);
     if (m) {
         const stream_id = Number.parseInt(m[1], 10);
         const sub = subs_by_stream_id.get(stream_id);