fixed parsing to support possible 3 or 4 digit versioning

.bumpversion.toml

@@ -8,11 +8,11 @@ parse = """(?x)
     (?P<patch>0|[1-9]\\d*)
     (?:
         \\.(?P<smallpatch>0|[1-9]\\d*)
-    )?
+    )?                                  # smallpatch section is optional
     (?:
-        (?P<pre_l>[a-zA-Z-]+)         # pre-release label
-        (?P<pre_n>[1-9]\\d*)        # pre-release version number
-    )?                                # pre-release section is optional
+        (?P<pre_l>[a-zA-Z-]+)           # pre-release label
+        (?P<pre_n>[1-9]\\d*)            # pre-release version number
+    )?                                  # pre-release section is optional
 """
 serialize = [
     "{major}.{minor}.{patch}{pre_l}{pre_n}",