[BUG][C#] IllegalArgumentException if string enum value ends with backslash

Bug Report Checklist

[x] Have you provided a full/minimal spec to reproduce the issue?
[x] Have you validated the input using an OpenAPI validator (example)?
[x] What's the version of OpenAPI Generator used?
[x] Have you search for related issues/PRs?
[x] What's the actual output vs expected output?
[ ] [Optional] Bounty to sponsor the fix (example)

Description

If a string enum value ends with a backslash, openapi-generator fails to generate code for csharp due to java.lang.IllegalArgumentException.

openapi-generator version

Tested with 4.2.2 and current master (2d24d42e65).

OpenAPI declaration file content or url

Example OpenAPI document

```json { "swagger": "2.0", "info": { "title": "Problematic enum strings example", "version": "1.0.0" }, "definitions": { "ErrorMessage": { "type": "string", "enum": [ "Error: Invalid character: \\", "RangeError: Too many items." ] } }, "paths": { "/count": { "get": { "operationId": "getCount", "responses": { "200": { "description": "Success", "schema": { "type": "integer" } }, "default": { "description": "Error", "schema": { "$ref": "#/definitions/ErrorMessage" } } } } } } } ```

Command line used for generation

openapi-generator generate -g csharp -i openapi.json -o generated

Steps to reproduce

Copy example OpenAPI document into openapi.json.
Run above command.

Observe output similar to the following:

[main] INFO  o.o.codegen.DefaultGenerator - OpenAPI Generator: csharp (client)
[main] INFO  o.o.codegen.DefaultGenerator - Generator 'csharp' is considered stable.
[main] INFO  o.o.c.l.AbstractCSharpCodegen - Environment variable CSHARP_POST_PROCESS_FILE not defined so the C# code may not be properly formatted by uncrustify (0.66 or later) or other code formatter. To define it, try `export CSHARP_POST_PROCESS_FILE="/usr/local/bin/uncrustify --no-backup" && export UNCRUSTIFY_CONFIG=/path/to/uncrustify-rules.cfg` (Linux/Mac). Note: replace /path/to with the location of uncrustify-rules.cfg
[main] INFO  o.o.c.l.AbstractCSharpCodegen - NOTE: To enable file post-processing, 'enablePostProcessFile' must be set to `true` (--enable-post-process-file for CLI).
[main] INFO  o.o.c.languages.CSharpClientCodegen - Generating code for .NET Framework v4.5
[main] WARN  o.o.codegen.utils.URLPathUtils - 'host' (OAS 2.0) or 'servers' (OAS 3.0) not defined in the spec. Default to [http://localhost] for server URL [http://localhost/]
[main] WARN  o.o.codegen.utils.URLPathUtils - 'host' (OAS 2.0) or 'servers' (OAS 3.0) not defined in the spec. Default to [http://localhost] for server URL [http://localhost/]
Exception in thread "main" java.lang.RuntimeException: Could not process model 'ErrorMessage'.Please make sure that your schema is correct!
    at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:472)
    at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:923)
    at org.openapitools.codegen.cmd.Generate.run(Generate.java:416)
    at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:61)
Caused by: java.lang.IllegalArgumentException: character to be escaped is missing
    at java.base/java.util.regex.Matcher.appendExpandedReplacement(Matcher.java:1020)
    at java.base/java.util.regex.Matcher.appendReplacement(Matcher.java:998)
    at java.base/java.util.regex.Matcher.replaceAll(Matcher.java:1182)
    at org.openapitools.codegen.utils.StringUtils.camelize(StringUtils.java:96)
    at org.openapitools.codegen.utils.StringUtils.camelize(StringUtils.java:55)
    at org.openapitools.codegen.languages.CSharpClientCodegen.toEnumVarName(CSharpClientCodegen.java:757)
    at org.openapitools.codegen.DefaultCodegen.postProcessModelsEnum(DefaultCodegen.java:438)
    at org.openapitools.codegen.languages.AbstractCSharpCodegen.postProcessModels(AbstractCSharpCodegen.java:405)
    at org.openapitools.codegen.DefaultGenerator.processModels(DefaultGenerator.java:1199)
    at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:467)
    ... 3 more

Related issues/PRs

None that I could find.

Suggest a fix

StringUtils.camelize could use Matcher.quoteReplacement on the replacement string.

Thanks for considering, Kevin

OpenAPITools / openapi-generator