is-crawler 1.4.3

Fast, regex-free crawler detection from user agents. Zero deps, ReDoS-safe heuristics, ~40× faster than alternatives.

is-crawler

Fast, regex-free crawler detection from user agents. Zero deps, ReDoS-safe heuristics, ~40× faster than alternatives.

Docs & live demo: is-crawler.tn3w.dev

Why regex-free?

Regex is a frequent source of ReDoS vulnerabilities, one un-anchored .* or nested quantifier against a hostile UA can spike CPU to seconds. Crawler detection runs on every request, so a catastrophic pattern is a denial-of-service primitive. is-crawler implements all heuristics with str.find + char scans. No regex engine, no backtracking, no ReDoS surface. crawler_info uses re only to match against curated DB patterns (monperrus/crawler-user-agents) which are simple literals (e.g. Googlebot\/, bingbot, AdsBot-Google([^-]|$), [wW]get), no nested quantifiers, no catastrophic backtracking paths.

Install

pip install is-crawler

Usage

from is_crawler import (
    is_crawler, crawler_signals, crawler_info, crawler_has_tag,
    crawler_name, crawler_version, crawler_url, CrawlerInfo,
)

ua = "Googlebot/2.1 (+http://www.google.com/bot.html)"

is_crawler(ua)                              # True
crawler_signals(ua)                         # ['bot_signal', 'no_browser_signature', 'url_in_ua']
crawler_name(ua)                            # 'Googlebot'
crawler_version(ua)                         # '2.1'
crawler_url(ua)                             # 'http://www.google.com/bot.html'

info = crawler_info(ua)                     # CrawlerInfo(...)
if info is not None:
    info.url                                # 'http://www.google.com/bot.html'
    info.description                        # "Google's main web crawling bot..."
    info.tags                               # ('search-engine',)

crawler_has_tag(ua, "search-engine")        # True
crawler_has_tag(ua, ["ai-crawler", "seo"])  # False

API

is_crawler(ua: str) -> bool

Heuristic detection. Returns True if the UA is a crawler. No DB lookup, no regex.

Three short-circuit rules:

1. Positive signal: bot keywords (bot, crawl, spider, scrape, headless, slurp, archiv, preview, ...), known tools (playwright, selenium, wget, lighthouse, sqlmap, nikto, nmap, httrack, pingdom, google-safety, ...), or a URL/email embedded in the UA.
2. No browser signature: missing Mozilla/, WebKit, Gecko, Trident, Presto, KHTML, Links, Lynx, Opera, or an OS token like (Windows, (Linux, (X11, (Macintosh.
3. Bare (compatible; ...): classic bot block without OS/browser tokens inside.

crawler_signals(ua: str) -> list[str]

Which individual rules fired. Subset of: bot_signal, no_browser_signature, bare_compatible, known_tool, url_in_ua. Useful for diagnostics and logging. is_crawler does not call this.

crawler_name(ua: str) -> str | None

Product name extracted from the UA.

• Googlebot/2.1 ... → 'Googlebot'
• Mozilla/5.0 (compatible; bingbot/2.0; ...) → 'bingbot'
• Mozilla/5.0 ... Speedy Spider (...) → 'Speedy Spider'
• Chrome/Firefox/Safari → None

crawler_version(ua: str) -> str | None

Version token extracted from the UA. Returns None if no non-browser version is detectable.

• curl/7.64.1 → '7.64.1'
• Mozilla/5.0 (compatible; Miniflux/2.0.10; ...) → '2.0.10'
• Googlebot/2.1 ... → '2.1'

crawler_url(ua: str) -> str | None

URL embedded in the UA (after +, ;, or -).

• Googlebot/2.1 (+http://www.google.com/bot.html) → 'http://www.google.com/bot.html'
• UA with no embedded URL → None

crawler_info(ua: str) -> CrawlerInfo | None

DB lookup against 646 known crawler patterns. Returns None for browsers (short-circuits via is_crawler).

class CrawlerInfo(NamedTuple):
    url: str                # crawler's info/docs URL (may be '')
    description: str        # human-readable description
    tags: tuple[str, ...]   # classification tags, e.g. ('search-engine',)

crawler_has_tag(ua: str, tags: str | Iterable[str]) -> bool

True if the crawler has any of the given tags. tags accepts a single string or a list.

Available tags: search-engine, ai-crawler, seo, social-preview, advertising, archiver, feed-reader, monitoring, scanner, academic, http-library, browser-automation.

Category shortcuts

One-tag wrappers over crawler_has_tag:

is_search_engine(ua)       # 'search-engine'
is_ai_crawler(ua)          # 'ai-crawler'
is_seo(ua)                 # 'seo'
is_social_preview(ua)      # 'social-preview'
is_advertising(ua)         # 'advertising'
is_archiver(ua)            # 'archiver'
is_feed_reader(ua)         # 'feed-reader'
is_monitoring(ua)          # 'monitoring'
is_scanner(ua)             # 'scanner'
is_academic(ua)            # 'academic'
is_http_library(ua)        # 'http-library'
is_browser_automation(ua)  # 'browser-automation'

is_good_crawler(ua) / is_bad_crawler(ua)

Opinionated groupings for quick allow/deny gates.

• Good (indexing, previews, archives, feeds, research): search-engine, social-preview, feed-reader, archiver, academic.
• Bad (scraping, scanning, unattributed traffic): ai-crawler, scanner, http-library, browser-automation, seo.

advertising and monitoring are intentionally neither: policy-dependent.

Middleware

from is_crawler import is_crawler, crawler_has_tag

@app.before_request
def gate():
    ua = request.headers.get("User-Agent", "")
    if crawler_has_tag(ua, "ai-crawler"):
        abort(403)
    if is_crawler(ua):
        log_crawler(ua)

robots.txt helpers

Generate directives from DB tags. Names extracted from DB patterns (slash/URL-only entries skipped).

from is_crawler import build_robots_txt, robots_agents_for_tags, iter_crawlers

robots_agents_for_tags("ai-crawler")
# ['AI2Bot', 'Applebot-Extended', 'Bytespider', 'CCBot', 'ChatGPT-User', 'Claude-Web', 'GPTBot', ...]

print(build_robots_txt(disallow=["ai-crawler", "scanner"]))
# User-agent: GPTBot
# Disallow: /
#
# User-agent: Nikto
# Disallow: /
# ...

build_robots_txt(allow="search-engine", path="/public")
# User-agent: Googlebot
# Allow: /public
# ...

for info, name in iter_crawlers():      # (CrawlerInfo, robots-name) per DB entry
    ...

CLI

python -m is_crawler "Googlebot/2.1 (+http://www.google.com/bot.html)"
tail -f access.log | awk -F'"' '{print $6}' | python -m is_crawler

One JSON object per UA (arg or stdin line) with is_crawler, name, version, url, signals, info.

Caching

Every public function has a 32k-entry LRU cache. Repeat UAs hit in ~40 ns.

Benchmarks

Python 3.14, Linux x86_64. Corpus: 1,231 crawler UAs, 15,812 browser UAs. cua = crawler-user-agents v1.44.

Hot-path (warm cache)

Function	is_crawler	cua	speedup
is_crawler (mixed)	0.05 µs	158.9 µs	3000×
crawler_info	0.60 µs	732.0 µs	1220×
crawler_signals	1.13 µs	-	-
crawler_name	0.33 µs	-	-
crawler_version	0.32 µs	-	-
crawler_url	0.09 µs	-	-
crawler_has_tag	0.10 µs	-	-

Cold-cache (per-call, no LRU hits)

Function	is_crawler	cua	speedup
is_crawler	4.67 µs	157.5 µs	34×
crawler_info	2.07 µs	733.4 µs	354×
crawler_name	1.36 µs	-	-
crawler_version	1.37 µs	-	-
crawler_url	0.29 µs	-	-

Cold-start

Module	Cold-start
is_crawler	1.29 ms
crawleruseragents	0.80 ms

DB patterns compile lazily per 48-entry chunk on first match.

Formatting

pip install black isort
isort . && black .
npx prtfm

License

Apache-2.0