[configuration] Fail on unsupported identifiers (#4002)

CHANGELOG.md

@@ -37,6 +37,8 @@ release.
 
 - Clarify syntax for environment variable substitution regular expression
   ([#4001](https://github.com/open-telemetry/opentelemetry-specification/pull/4001))
+- Error out on invalid identifiers in environment variable substitution.
+  ([#4002](https://github.com/open-telemetry/opentelemetry-specification/pull/4002))
 
 ### Common
 

specification/configuration/file-configuration.md

@@ -88,6 +88,15 @@ more non line break characters (i.e. any character except `\n`). If a referenced
 environment variable is not defined and does not have a `DEFAULT_VALUE`, it MUST
 be replaced with an empty value.
 
+When parsing a configuration file that contains a reference not matching
+the references regular expression but does match the following PCRE2
+regular expression, the parser MUST return an empty result (no partial
+results are allowed) and an error describing the parse failure to the user.
+
+```regexp
+\$\{(?<INVALID_IDENTIFIER>[^}]+)\}
+```
+
 Node types MUST be interpreted after environment variable substitution takes
 place. This ensures the environment string representation of boolean, integer,
 or floating point fields can be properly converted to expected types.
@@ -125,6 +134,7 @@ string_key_with_default: ${UNDEFINED_KEY:-fallback}   # UNDEFINED_KEY is not def
 undefined_key: ${UNDEFINED_KEY}                       # Invalid reference, UNDEFINED_KEY is not defined and is replaced with ""
 ${STRING_VALUE}: value                                # Invalid reference, substitution is not valid in mapping keys and reference is ignored
 recursive_key: ${REPLACE_ME}                          # Valid reference to REPLACE_ME
+# invalid_identifier_key: ${STRING_VALUE:?error}      # If uncommented, this is an invalid identifier, it would fail to parse
 ```
 
 Environment variable substitution results in the following YAML: