diff --git a/packaging/_tokenizer.py b/packaging/_tokenizer.py
index da6e7b06..de389cb3 100644
--- a/packaging/_tokenizer.py
+++ b/packaging/_tokenizer.py
@@ -53,12 +53,12 @@ def __str__(self) -> str:
         re.VERBOSE,
     ),
     "OP": r"(===|==|~=|!=|<=|>=|<|>)",
-    "BOOLOP": r"(or|and)",
-    "IN": r"in",
-    "NOT": r"not",
+    "BOOLOP": r"\b(or|and)\b",
+    "IN": r"\bin\b",
+    "NOT": r"\bnot\b",
     "VARIABLE": re.compile(
         r"""
-            (
+            \b(
                 python_version
                 |python_full_version
                 |os[._]name
@@ -68,14 +68,14 @@ def __str__(self) -> str:
                 |python_implementation
                 |implementation_(name|version)
                 |extra
-            )
+            )\b
         """,
         re.VERBOSE,
     ),
     "VERSION": re.compile(Specifier._version_regex_str, re.VERBOSE | re.IGNORECASE),
     "AT": r"\@",
     "URL": r"[^ \t]+",
-    "IDENTIFIER": r"[a-zA-Z0-9][a-zA-Z0-9._-]*",
+    "IDENTIFIER": r"\b[a-zA-Z0-9][a-zA-Z0-9._-]*\b",
     "WS": r"[ \t]+",
     "END": r"$",
 }
diff --git a/tests/test_requirements.py b/tests/test_requirements.py
index 86516fd7..93839d69 100644
--- a/tests/test_requirements.py
+++ b/tests/test_requirements.py
@@ -390,9 +390,27 @@ def test_error_invalid_marker_notin_without_whitespace(self) -> None:
         # THEN
         assert ctx.exconly() == (
             "packaging.requirements.InvalidRequirement: "
-            "Expected whitespace after 'not'\n"
+            "Expected marker operator, one of <=, <, !=, ==, >=, >, ~=, ===, "
+            "not, not in\n"
             "    name; '3.7' notin python_version\n"
-            "                   ^"
+            "                ^"
+        )
+
+    def test_error_when_no_word_boundary(self) -> None:
+        # GIVEN
+        to_parse = "name; '3.6'inpython_version"
+
+        # WHEN
+        with pytest.raises(InvalidRequirement) as ctx:
+            Requirement(to_parse)
+
+        # THEN
+        assert ctx.exconly() == (
+            "packaging.requirements.InvalidRequirement: "
+            "Expected marker operator, one of <=, <, !=, ==, >=, >, ~=, ===, "
+            "not, not in\n"
+            "    name; '3.6'inpython_version\n"
+            "               ^"
         )
 
     def test_error_invalid_marker_not_without_in(self) -> None: