When using FeignClient, an error occurs when a value with a special structure is passed in the header map

[klook-tech-tonyke]

feign-core:11.8
When the content of my header map is as follows, the following exception is throw
headerMap：
{
"x-test":{"test":["123:861"]}
}
error:
java.util.regex.PatternSyntaxException: Unclosed character class near index 4
["123
^
at java.base/java.util.regex.Pattern.error(Pattern.java:2204)
at java.base/java.util.regex.Pattern.clazz(Pattern.java:2869)
at java.base/java.util.regex.Pattern.sequence(Pattern.java:2315)
at java.base/java.util.regex.Pattern.expr(Pattern.java:2245)
at java.base/java.util.regex.Pattern.compile(Pattern.java:1945)
at java.base/java.util.regex.Pattern.(Pattern.java:1576)
at java.base/java.util.regex.Pattern.compile(Pattern.java:1101)
at feign.template.Expression.lambda$new$0(Expression.java:35)
at java.base/java.util.Optional.ifPresent(Optional.java:178)
at feign.template.Expression.(Expression.java:35)
at feign.template.Expressions$SimpleExpression.(Expressions.java:106)
at feign.template.Expressions.create(Expressions.java:86)
at feign.template.Template.parseFragment(Template.java:218)
at feign.template.Template.parseTemplate(Template.java:202)
at feign.template.Template.(Template.java:61)
at feign.template.HeaderTemplate.(HeaderTemplate.java:82)
at feign.template.HeaderTemplate.create(HeaderTemplate.java:48)
at feign.RequestTemplate.lambda$appendHeader$3(RequestTemplate.java:748)
at java.base/java.util.TreeMap.callRemappingFunctionWithCheck(TreeMap.java:792)
at java.base/java.util.TreeMap.compute(TreeMap.java:722)
at feign.RequestTemplate.appendHeader(RequestTemplate.java:746)
at feign.RequestTemplate.header(RequestTemplate.java:708)
at feign.ReflectiveFeign$BuildTemplateByResolvingArgs.addHeaderMapHeaders(ReflectiveFeign.java:294)
at feign.ReflectiveFeign$BuildTemplateByResolvingArgs.create(ReflectiveFeign.java:243)
at feign.SynchronousMethodHandler.invoke(SynchronousMethodHandler.java:84)
at feign.ReflectiveFeign$FeignInvocationHandler.invoke(ReflectiveFeign.java:100)
at jdk.proxy2/jdk.proxy2.$Proxy144.getOrderDetail(Unknown Source)

[velo]

Thanks for the clear stack trace. This looks like header-map values containing brackets/colons are being parsed through the template expression path and can trip PatternSyntaxException, so I've labeled it as a bug.

A good next step would be a minimal reproducer against a current Feign version plus a PR with a regression test around RequestTemplate/header-map handling. I'll review a compatible fix that preserves existing template behavior.