diff --git a/src/poetry/core/factory.py b/src/poetry/core/factory.py
index 1d56d78c7..0a776659c 100644
--- a/src/poetry/core/factory.py
+++ b/src/poetry/core/factory.py
@@ -409,6 +409,22 @@ def validate(
                             'Use "allow-prereleases" instead.'
                         )
 
+            if "extras" in config:
+                for extra_name, requirements in config["extras"].items():
+                    extra_name = canonicalize_name(extra_name)
+
+                    for req in requirements:
+                        req_name = canonicalize_name(req)
+                        for dependency in config.get("dependencies", {}).keys():
+                            dep_name = canonicalize_name(dependency)
+                            if req_name == dep_name:
+                                break
+                        else:
+                            result["errors"].append(
+                                f'Cannot find dependency "{req}" for extra '
+                                f'"{extra_name}" in main dependencies.'
+                            )
+
             # Checking for scripts with extras
             if "scripts" in config:
                 scripts = config["scripts"]
diff --git a/src/poetry/core/json/schemas/poetry-schema.json b/src/poetry/core/json/schemas/poetry-schema.json
index a3a09738c..9b39a1047 100644
--- a/src/poetry/core/json/schemas/poetry-schema.json
+++ b/src/poetry/core/json/schemas/poetry-schema.json
@@ -155,7 +155,8 @@
         "^[a-zA-Z-_.0-9]+$": {
           "type": "array",
           "items": {
-            "type": "string"
+            "type": "string",
+            "pattern": "^[a-zA-Z-_.0-9]+$"
           }
         }
       }
diff --git a/tests/fixtures/project_failing_strict_validation/pyproject.toml b/tests/fixtures/project_failing_strict_validation/pyproject.toml
index 6d282ba97..3fc8d4fbb 100644
--- a/tests/fixtures/project_failing_strict_validation/pyproject.toml
+++ b/tests/fixtures/project_failing_strict_validation/pyproject.toml
@@ -5,6 +5,9 @@ readme = ["README.rst", "README_WITH_ANOTHER_EXTENSION.md"]
 python = "*"
 pathlib2 = { version = "^2.2", python = "3.7", allows-prereleases = true }
 
+[tool.poetry.extras]
+some_extras = ["missing_extra", "another_missing_extra"]
+
 [tool.poetry.scripts]
 a_script_with_unknown_extra = { reference = "a_script_with_unknown_extra.py", type = "file", extras = ["foo"] }
 a_script_without_extras = { reference = "a_script_without_extras.py", type = "file" }
diff --git a/tests/json/test_poetry_schema.py b/tests/json/test_poetry_schema.py
index bb6399abb..b2409b56d 100644
--- a/tests/json/test_poetry_schema.py
+++ b/tests/json/test_poetry_schema.py
@@ -65,3 +65,15 @@ def test_multiline_description(
 
     regex = r"\\A[^\n]*\\Z"
     assert errors[0] == f"[description] {bad_description!r} does not match '{regex}'"
+
+
+def test_bad_extra(base_object: dict[str, Any]) -> None:
+    bad_extra = "a{[*+"
+    base_object["extras"] = {}
+    base_object["extras"]["test"] = [bad_extra]
+
+    errors = validate_object(base_object, "poetry-schema")
+    assert len(errors) == 1
+    assert (
+        errors[0] == f"[extras.test.0] {bad_extra!r} does not match '^[a-zA-Z-_.0-9]+$'"
+    )
diff --git a/tests/test_factory.py b/tests/test_factory.py
index af4101dd0..1a5f23091 100644
--- a/tests/test_factory.py
+++ b/tests/test_factory.py
@@ -232,6 +232,14 @@ def test_validate_strict_fails_strict_and_non_strict() -> None:
             "'version' is a required property",
             "'description' is a required property",
             "'authors' is a required property",
+            (
+                'Cannot find dependency "missing_extra" for extra "some-extras" in '
+                "main dependencies."
+            ),
+            (
+                'Cannot find dependency "another_missing_extra" for extra '
+                '"some-extras" in main dependencies.'
+            ),
             (
                 'Script "a_script_with_unknown_extra" requires extra "foo" which is not'
                 " defined."