fix(core): support unbraced $name fields in $ style formats

docs/changelog.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 - Logging a `dict` no longer modifies it. `exc_info` and `stack_info` were previously added to the caller's `dict`. [#66](https://github.com/nhairs/python-json-logger/pull/66)
+- `$` style formats now support unbraced `$name` fields, not just `${name}`. [#18](https://github.com/nhairs/python-json-logger/issues/18)
 
 Thanks @gaoflow
 

src/pythonjsonlogger/core.py

@@ -61,7 +61,9 @@
     RESERVED_ATTRS.sort()
 
 
-STYLE_STRING_TEMPLATE_REGEX = re.compile(r"\$\{(.+?)\}", re.IGNORECASE)  # $ style
+STYLE_STRING_TEMPLATE_REGEX = re.compile(
+    r"\$(?:\$|\{(?P<braced>.+?)\}|(?P<named>[_a-z][_a-z0-9]*))", re.IGNORECASE
+)  # $ style
 STYLE_STRING_FORMAT_REGEX = re.compile(r"\{(.+?)\}", re.IGNORECASE)  # { style
 STYLE_PERCENT_REGEX = re.compile(r"%\((.+?)\)", re.IGNORECASE)  # % style
 
@@ -302,7 +304,12 @@ def parse(self) -> list[str]:
             raise ValueError(f"Style {self._style!r} is not supported")
 
         if isinstance(self._style, logging.StringTemplateStyle):
-            formatter_style_pattern = STYLE_STRING_TEMPLATE_REGEX
+            # String templates support both ${name} and $name, and $$ is an escaped literal
+            return [
+                match.group("braced") or match.group("named")
+                for match in STYLE_STRING_TEMPLATE_REGEX.finditer(self._fmt)
+                if match.group("braced") or match.group("named")
+            ]
 
         elif isinstance(self._style, logging.StrFormatStyle):
             formatter_style_pattern = STYLE_STRING_FORMAT_REGEX

tests/test_formatters.py

@@ -167,6 +167,22 @@ def test_percentage_format(env: LoggingEnvironment, class_: type[BaseJsonFormatt
     return
 
 
+@pytest.mark.parametrize("class_", ALL_FORMATTERS)
+def test_string_template_format(env: LoggingEnvironment, class_: type[BaseJsonFormatter]):
+    # Note: string templates support both $name and ${name}, and $$ is an escaped literal $
+    env.set_formatter(
+        class_("$$literal $levelname ${message} $filename ${lineno} $asctime", style="$")
+    )
+
+    msg = "testing logging format"
+    env.logger.info(msg)
+    log_json = env.load_json()
+
+    assert log_json["message"] == msg
+    assert log_json.keys() == {"levelname", "message", "filename", "lineno", "asctime"}
+    return
+
+
 @pytest.mark.parametrize("class_", ALL_FORMATTERS)
 def test_comma_format(env: LoggingEnvironment, class_: type[BaseJsonFormatter]):
     # Note: we have double comma `,,` to test handling "empty" names