User Agent Parser

About User Agent Parser

What the Parser Detects

Five properties are extracted by matching well-known tokens in the UA string against regular expressions.

Worked Example: Parsing a Chrome-on-Windows String

Take the following real UA string: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/124.0.0.0 Safari/537.36.

Step 1, browser detection. The parser looks for Edg/ first (Edge impersonates Chrome, so Edge must win), then OPR/, Firefox/, Safari/ without Chrome/, and finally Chrome/. The string contains Chrome/124.0.0.0 and no Edg/ or OPR/, so the result is Chrome 124.0.0.0.

Step 2, OS detection. Windows NT 10.0 maps to Windows 10 or 11. Microsoft kept Windows NT 10.0 for Windows 11 rather than bumping it to 11, so the parser reports "Windows 10/11" and servers that need the distinction must use the Sec-CH-UA-Platform-Version client hint.

Step 3, engine. AppleWebKit/537.36 combined with Chrome/ means Blink (Chromium's fork of WebKit). AppleWebKit alone, without Chrome, means real WebKit (Safari).

Step 4, device. No Mobile or Android token, no bot signature, so the device is Desktop. Final parsed output: Chrome 124, Windows 10/11, Blink, Desktop.

Now try a tricky one: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/124.0.0.0 Safari/537.36 Edg/124.0.2478.67. This is Edge, not Chrome. A parser that checks Chrome/ first would misreport it. The rule is: check the most specific brand first (Edg, OPR, Vivaldi, Brave, then Firefox, then Safari-without-Chrome, then Chrome as the fallback). The regex order in this tool's source follows that rule exactly.

A third example from an iPhone: Mozilla/5.0 (iPhone; CPU iPhone OS 17_2 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17.2 Mobile/15E148 Safari/604.1. The iPhone token sets the device to Mobile, iPhone OS 17_2 becomes iOS 17.2 (underscores swapped for dots), and Safari wins the browser check because the string does not contain Chrome/. Apple uses underscores in OS versions because dots break old regex patterns, so parsers must normalise them back.

Why User Agent Strings Are So Long

Modern UA strings carry decades of compatibility baggage because every new browser had to pretend to be the previous dominant one to avoid being served degraded content. The Chrome string above references Mozilla, WebKit, KHTML, Gecko, and Safari - five different product names for one browser.

Each new browser inherited the previous one's claims rather than dropping them, because thousands of legacy sites still key detection off old strings. The string is effectively a fossil record of browser history.

What Is the Current Browser Market Share?

Chrome sits at 71.37% global share in 2026, according to Statcounter Global Stats, followed by Safari at 14.75%, Edge at 4.65%, and Firefox at 2.25%. On desktop Chrome pushes past 76% and Edge takes second at roughly 9%. On mobile Chrome holds 64.7% while Safari dominates iOS traffic. Because four of the top five browsers (Chrome, Edge, Opera, Samsung Internet) run Blink, roughly 70%+ of real-world traffic originates from a Chromium-family engine.

On iOS, until iOS 17.4 in March 2024, Apple's App Store guidelines forced all browsers to use WebKit under the hood. Chrome on iOS was a WebKit browser with Chrome's UI. In the EU, iOS 17.4 opened the door to non-WebKit engines under the Digital Markets Act, though adoption has been slow.

Common Bot and Crawler User Agents

Most bots identify themselves honestly in the UA header because search engines and AI crawlers want webmasters to allow or block them deliberately via robots.txt. Here are the signatures to recognise.

Malicious scraping bots typically forge a real browser's UA string to blend in. UA-based blocking alone is not a security control - rate limiting, TLS fingerprinting (JA3/JA4), and behavioural analysis catch far more. If you are writing server-side detection for legitimate traffic, the HTTP Status Codes reference pairs well with UA analysis for understanding what your server is returning to each crawler.

Client Hints - The Modern Alternative

User-Agent Client Hints (UA-CH) replace UA parsing with structured HTTP headers the browser sends only when the server asks for them, per the W3C WICG specification. The server opts in by sending an Accept-CH response header listing which hints it wants, and the browser includes those hints on subsequent requests.

Chrome began User-Agent Reduction in Q2 2022 (starting with Chrome 101) and completed it across all platforms by 2023. The traditional UA string still works but now returns a frozen minor version and a unified OS version. For example, every Chrome user on Windows now reports Windows NT 10.0 regardless of whether they run Windows 10 or 11. To get the real OS version, servers must request Sec-CH-UA-Platform-Version explicitly. Firefox and Safari have not implemented UA-CH as of 2026 and continue to rely on the traditional string.

Reading a UA String Token by Token

Every modern UA string follows the same top-level structure: Mozilla/5.0 (platform info) engine (compatibility note) browser-and-version. Knowing the layout makes manual inspection faster when regex libraries are overkill.

For regex-based parsing or writing your own matcher, the Regex Tester is the fastest way to validate each pattern against the real UAs in your access logs.

Common Parsing Mistakes

The biggest mistake is checking for Chrome before Edg or OPR. Edge and Opera both contain Chrome/ in their UA strings, so naive code misidentifies them. Always check for the more specific brand first. The second mistake is treating Safari/ as evidence of Safari - Chrome includes Safari/537.36 for historical compatibility, so Safari detection must check for Safari/ AND NOT Chrome/.

A third pitfall: on iOS pre-17.4, every browser reports the same WebKit UA with minor variations, so Chrome on iPhone looks almost identical to Safari on iPhone. Device-specific feature detection (via navigator properties or the Screen Resolution Checker) is more reliable than UA parsing when precise browser identity matters.

Sources

• Statcounter Global Stats - Browser Market Share (2026)
• Chrome for Developers - User-Agent Client Hints
• Google Privacy Sandbox - User-Agent Reduction
• MDN - User-Agent Reduction
• W3C WICG - User-Agent Client Hints Specification