diff --git a/maigret/utils.py b/maigret/utils.py
index e980a6c..7b215ee 100644
--- a/maigret/utils.py
+++ b/maigret/utils.py
@@ -48,7 +48,7 @@ def enrich_link_str(link: str) -> str:
 
 
 class URLMatcher:
-    _HTTP_URL_RE_STR = "^https?://(www.|m.)?(.+)$"
+    _HTTP_URL_RE_STR = r"^https?://(www\.|m\.)?(.+)$"
     HTTP_URL_RE = re.compile(_HTTP_URL_RE_STR)
     UNSAFE_SYMBOLS = ".?"
 
diff --git a/tests/test_sites.py b/tests/test_sites.py
index a3bce6a..78b07ba 100644
--- a/tests/test_sites.py
+++ b/tests/test_sites.py
@@ -160,7 +160,7 @@ def test_site_url_detector():
 
     assert (
         db.sites[0].url_regexp.pattern
-        == r'^https?://(www.|m.)?forum\.amperka\.ru/members/\?username=(.+?)$'
+        == r'^https?://(www\.|m\.)?forum\.amperka\.ru/members/\?username=(.+?)$'
     )
     assert (
         db.sites[0].detect_username('http://forum.amperka.ru/members/?username=test')
diff --git a/tests/test_utils.py b/tests/test_utils.py
index 26ca1cd..b606f03 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -123,6 +123,17 @@ def test_url_extract_main_part():
         assert not url_regexp.match(url) is None
 
 
+def test_url_extract_main_part_keeps_host_starting_with_prefix_letters():
+    # The optional (www.|m.)? group must only strip the literal 'www.'/'m.'
+    # subdomain prefixes. With unescaped dots it instead ate the first
+    # character(s) of any host starting with 'm'/'www' (e.g. medium.com).
+    assert URLMatcher.extract_main_part('https://medium.com/alice') == 'medium.com/alice'
+    assert URLMatcher.extract_main_part('https://myspace.com/bob') == 'myspace.com/bob'
+    # genuine mobile/web subdomain prefixes are still stripped
+    assert URLMatcher.extract_main_part('https://m.wikipedia.org/wiki/Foo') == 'wikipedia.org/wiki/Foo'
+    assert URLMatcher.extract_main_part('https://www.flickr.com/photos/x') == 'flickr.com/photos/x'
+
+
 def test_url_make_profile_url_regexp():
     url_main_part = 'flickr.com/photos/{username}'
 
@@ -139,7 +150,7 @@ def test_url_make_profile_url_regexp():
         # ensure all combinations match pattern
         assert (
             URLMatcher.make_profile_url_regexp(url).pattern
-            == r'^https?://(www.|m.)?flickr\.com/photos/(.+?)$'
+            == r'^https?://(www\.|m\.)?flickr\.com/photos/(.+?)$'
         )