Support string patterns in config fields

Amp-Thread-ID: https://ampcode.com/threads/T-019e665f-205b-755b-ab28-497399c0e46d
Co-authored-by: Amp <amp@ampcode.com>

Author: marcleblanc2

README.md

@@ -87,8 +87,8 @@ Config precedence is: code defaults, `.env`, shell environment, then CLI
 overrides. API client modules can provide shared Config base classes such as
 `LinearClientConfig`, and `parse_args` resolves `op://...` references by
 default. `config_field(default=...)` supports aliases, store-true /
-store-false command flags, optional values, and numeric bounds for simple
-CLIs. Pass a custom `argparse.ArgumentParser` to `parse_args` only when you
+store-false command flags, optional values, numeric bounds, and string patterns
+for simple CLIs. Pass a custom `argparse.ArgumentParser` to `parse_args` only when you
 need parsing beyond Config fields. Help text preserves description and
 argument-help newlines, and reserves enough option-column width for long config
 flags. Mark sensitive fields with `secret=True` so snapshots do not expose

src/src_py_lib/utils/config.py

@@ -94,6 +94,7 @@ def config_field(
     ge: int | float | None = None,
     lt: int | float | None = None,
     le: int | float | None = None,
+    pattern: str | None = None,
 ) -> Any:
     """Return a Pydantic field with Config environment and CLI metadata."""
     option = ConfigOption(
@@ -121,6 +122,8 @@ def config_field(
         field_kwargs["lt"] = lt
     if le is not None:
         field_kwargs["le"] = le
+    if pattern is not None:
+        field_kwargs["pattern"] = pattern
     return Field(default, **field_kwargs)
 
 

tests/test_logging_http_clients.py

@@ -219,6 +219,18 @@ class BoundedConfig(Config):
     )
 
 
+class PatternConfig(Config):
+    """Config model with a string pattern constraint."""
+
+    date: str | None = config_field(
+        default=None,
+        env_var="PATTERN_DATE",
+        cli_flag="--date",
+        metavar="YYYY-MM-DD",
+        pattern=r"^\d{4}-\d{2}-\d{2}$",
+    )
+
+
 class CommandStyleConfig(Config):
     """Config model with command-style flags."""
 
@@ -489,6 +501,30 @@ def test_config_field_supports_numeric_bounds(self) -> None:
                 resolve_op_refs=False,
             )
 
+    def test_config_field_supports_string_pattern(self) -> None:
+        config = load_config(
+            PatternConfig,
+            env_file=None,
+            env={"PATTERN_DATE": "2026-01-31"},
+            resolve_op_refs=False,
+        )
+
+        self.assertEqual(config.date, "2026-01-31")
+        with self.assertRaisesRegex(ConfigError, "String should match pattern"):
+            load_config(
+                PatternConfig,
+                env_file=None,
+                env={"PATTERN_DATE": "2026-1-31"},
+                resolve_op_refs=False,
+            )
+        with self.assertRaisesRegex(ConfigError, "String should match pattern"):
+            load_config(
+                PatternConfig,
+                env_file=None,
+                env={"PATTERN_DATE": "2026-01-31T00:00:00Z"},
+                resolve_op_refs=False,
+            )
+
     def test_logging_config_mixin_adds_log_level_from_cli_and_env(self) -> None:
         parser = argparse.ArgumentParser()
         add_config_arguments(parser, LoggingExampleConfig)