OpenAPITools/openapi-generator · error · IllegalArgumentException

Pattern must follow the Perl /pattern/modifiers convention.

Error message

Pattern must follow the Perl /pattern/modifiers convention. {pattern} is not valid.

What it means

AbstractPythonPydanticV1Codegen.postProcessPattern converts each schema pattern into Python regex pieces and enforces Perl /pattern/modifiers form: first character '/' and last '/' at index >= 2, else IllegalArgumentException. Since the pipeline's addRegularExpressionDelimiter only auto-wraps patterns that do NOT already start with '/', the throw fires when a spec pattern begins with '/' but carries no non-empty delimited body.

Source

Thrown at modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractPythonPydanticV1Codegen.java:1922

     * The OpenAPI pattern spec follows the Perl convention and style of modifiers. Python
     * does not support this in as natural a way so it needs to convert it. See
     * https://docs.python.org/2/howto/regex.html#compilation-flags for details.
     *
     * @param pattern (the String pattern to convert from python to Perl convention)
     * @param vendorExtensions (list of custom x-* properties for extra functionality-see https://swagger.io/docs/specification/openapi-extensions/)
     * @return void
     * @throws IllegalArgumentException if pattern does not follow the Perl /pattern/modifiers convention
     *
     * Includes fix for issue #6675
     */
    public void postProcessPattern(String pattern, Map<String, Object> vendorExtensions) {
        if (pattern != null) {
            int i = pattern.lastIndexOf('/');

            // TODO update the check below follow python convention
            //Must follow Perl /pattern/modifiers convention
            if (pattern.charAt(0) != '/' || i < 2) {
                throw new IllegalArgumentException("Pattern must follow the Perl "
                        + "/pattern/modifiers convention. " + pattern + " is not valid.");
            }

            String regex = pattern.substring(1, i).replace("'", "\\'");
            List<String> modifiers = new ArrayList<String>();

            for (char c : pattern.substring(i).toCharArray()) {
                if (regexModifiers.containsKey(c)) {
                    String modifier = regexModifiers.get(c);
                    modifiers.add(modifier);
                }
            }

            vendorExtensions.put(X_REGEX, regex.replace("\"", "\\\""));
            vendorExtensions.put(X_PATTERN, pattern.replace("\"", "\\\""));
            vendorExtensions.put(X_MODIFIERS, modifiers);
        }
    }

View on GitHub (pinned to fcec517be3)

Solutions

1. Rewrite as a bare ECMAScript regex without delimiters, e.g. "^/api/v[0-9]+$"
2. Or use a complete Perl form with body and flags, e.g. "/^\d{4}$/i"
3. Strip slash-only placeholder patterns from the spec

Example fix

# before
pattern: /api/v[0-9]+

# after
pattern: ^/api/v[0-9]+

// JS: same pre-scan as other Python generators
function badPattern(p) {
  if (!p || !p.startsWith('/')) return false;
  return p.lastIndexOf('/') < 2;
}

try { generator.generate(); } catch (IllegalArgumentException e) { if (e.getMessage().contains("Perl")) { /* rewrite the offending pattern without delimiters */ } throw e; }

When it happens

Trigger: Generating -g python-pydantic-v1 with a schema pattern like "/", "//", or "/api/v[0-9]+" — a leading slash with no closing delimiter. Plain "^\d+$" style patterns are wrapped automatically and pass.

Common situations: Path-matching regexes starting with a literal slash; partially converted Perl-notation patterns; placeholder slash-only patterns left in specs.

Related errors

• Pattern must follow the Perl /pattern/modifiers convention.
• Pattern must follow the Perl /pattern/modifiers convention.
• Error! Codegen Property not yet supported in getPydanticType
• Error! Failed to process getPydanticType when getting the co
• Error! Codegen Parameter not yet supported in getPydanticTyp

AI-assisted analysis of OpenAPITools/openapi-generator@fcec517be3 (2026-08-22). Data as JSON: /api/errors/49d6a80d310d1cd4. Report an issue: GitHub.