fix #838: handle incorrectly passed empty tag regex as default

3 files changed, 31 insertions, 8 deletions

diff --git a/src/setuptools_scm/__init__.py b/src/setuptools_scm/__init__.py
index 96c8349..faea673 100644
--- a/src/setuptools_scm/__init__.py
+++ b/src/setuptools_scm/__init__.py
@@ -6,6 +6,7 @@ from __future__ import annotations
 
 import os
 import re
+import warnings
 from typing import Any
 from typing import Pattern
 from typing import TYPE_CHECKING
@@ -133,7 +134,15 @@ def get_version(
     version_cls = _validate_version_cls(version_cls, normalize)
     del normalize
     if isinstance(tag_regex, str):
-        tag_regex = re.compile(tag_regex)
+        if tag_regex == "":
+            warnings.warn(
+                DeprecationWarning(
+                    "empty regex for tag regex is invalid, using default"
+                )
+            )
+            tag_regex = DEFAULT_TAG_REGEX
+        else:
+            tag_regex = re.compile(tag_regex)
     config = Configuration(**locals())
     maybe_version = _get_version(config)
 
diff --git a/src/setuptools_scm/version.py b/src/setuptools_scm/version.py
index 128533c..8cf48e8 100644
--- a/src/setuptools_scm/version.py
+++ b/src/setuptools_scm/version.py
@@ -38,10 +38,8 @@ SEMVER_LEN = 3
 def _parse_version_tag(
     tag: str | object, config: _config.Configuration
 ) -> dict[str, str] | None:
-    tagstring = tag if isinstance(tag, str) else str(tag)
-    match = config.tag_regex.match(tagstring)
+    match = config.tag_regex.match(str(tag))
 
-    result = None
     if match:
         key: str | int
         if len(match.groups()) == 1:
@@ -49,14 +47,23 @@ def _parse_version_tag(
         else:
             key = "version"
 
+        full = match.group(0)
+        log.debug("%r %r %s", tag, config.tag_regex, match)
+        log.debug(
+            "key %s data %s, %s, %r", key, match.groupdict(), match.groups(), full
+        )
         result = {
             "version": match.group(key),
-            "prefix": match.group(0)[: match.start(key)],
-            "suffix": match.group(0)[match.end(key) :],
+            "prefix": full[: match.start(key)],
+            "suffix": full[match.end(key) :],
         }
 
-    log.debug(f"tag '{tag}' parsed to {result}")
-    return result
+        log.debug("tag %r parsed to %r", tag, result)
+        return result
+    else:
+        log.debug("tag %r did not parse", tag)
+
+        return None
 
 
 def callable_or_entrypoint(group: str, callable_or_name: str | Any) -> Any:
diff --git a/testing/test_basic_api.py b/testing/test_basic_api.py
index f1b4c4f..6fd39bd 100644
--- a/testing/test_basic_api.py
+++ b/testing/test_basic_api.py
@@ -121,6 +121,13 @@ setup(name="myscm", use_scm_version={"fallback_version": "12.34"})
     assert res.stdout == "12.34"
 
 
+def test_get_version_blank_tag_regex() -> None:
+    with pytest.warns(
+        DeprecationWarning, match="empty regex for tag regex is invalid, using default"
+    ):
+        setuptools_scm.get_version(tag_regex="")
+
+
 @pytest.mark.parametrize(
     "version", ["1.0", "1.2.3.dev1+ge871260", "1.2.3.dev15+ge871260.d20180625", "2345"]
 )