JMeter JSON Assertion: Path must not end with a '.' or '..' - json

Question

I'm trying to replicate the JMeter script described here: https://www.blazemeter.com/blog/selenium-vs-jmeter-which-one-should-you-choose-and-when -- specifically under the "Front End: Testing Custom Communication Channels".
The author talks about setting up JSON assertions to verify response data, which is where I'm stuck. I'll post my code at the end, but what I've done is setup a HTTP request to POST to https:\reqres.in\api\users, sending the JSON payload shown in the article:
{
"name": "new_user",
"job": "user_role"
}
I then have the HTTP Header Manager setup with the header "Content-Type" set to "application/JSON". Finally, there's a JSON response assertion I tried to setup to replicate the author's description. I checked the box for "Match as regular expression" and set the expected value as [0-9]+. (I'm guessing this is where I went horribly wrong.)
The script's view results tree listener shows that the script did get a response:
{"name":"new_user","job":"user_role","id":"86","createdAt":"2020-11-16T23:41:04.645Z"}
But the script failed because the assertion failed with this message:
Assertion error: false
Assertion failure: true Assertion failure
message: Path must not end with a '.' or '..'
Here's my full script:
<?xml version="1.0" encoding="UTF-8"?>
<jmeterTestPlan version="1.2" properties="5.0" jmeter="5.1 r1853635">
<hashTree>
<TestPlan guiclass="TestPlanGui" testclass="TestPlan" testname="Test Plan" enabled="true">
<stringProp name="TestPlan.comments"></stringProp>
<boolProp name="TestPlan.functional_mode">false</boolProp>
<boolProp name="TestPlan.serialize_threadgroups">false</boolProp>
<elementProp name="TestPlan.user_defined_variables" elementType="Arguments" guiclass="ArgumentsPanel" testclass="Arguments" testname="User Defined Variables" enabled="true">
<collectionProp name="Arguments.arguments"/>
</elementProp>
<stringProp name="TestPlan.user_define_classpath"></stringProp>
</TestPlan>
<hashTree>
<Arguments guiclass="ArgumentsPanel" testclass="Arguments" testname="User Defined Variables" enabled="true">
<collectionProp name="Arguments.arguments">
<elementProp name="host" elementType="Argument">
<stringProp name="Argument.name">host</stringProp>
<stringProp name="Argument.value">wsf.cdyne.com</stringProp>
<stringProp name="Argument.metadata">=</stringProp>
<stringProp name="Argument.desc">Host of Webservice</stringProp>
</elementProp>
</collectionProp>
</Arguments>
<hashTree/>
<ConfigTestElement guiclass="HttpDefaultsGui" testclass="ConfigTestElement" testname="HTTP Request Defaults" enabled="true">
<elementProp name="HTTPsampler.Arguments" elementType="Arguments" guiclass="HTTPArgumentsPanel" testclass="Arguments" testname="User Defined Variables" enabled="true">
<collectionProp name="Arguments.arguments"/>
</elementProp>
<stringProp name="HTTPSampler.domain">${host}</stringProp>
<stringProp name="HTTPSampler.port"></stringProp>
<stringProp name="HTTPSampler.protocol"></stringProp>
<stringProp name="HTTPSampler.contentEncoding"></stringProp>
<stringProp name="HTTPSampler.path"></stringProp>
<stringProp name="HTTPSampler.concurrentPool">4</stringProp>
<stringProp name="HTTPSampler.connect_timeout"></stringProp>
<stringProp name="HTTPSampler.response_timeout"></stringProp>
</ConfigTestElement>
<hashTree/>
<ThreadGroup guiclass="ThreadGroupGui" testclass="ThreadGroup" testname="Number of Users" enabled="true">
<stringProp name="ThreadGroup.on_sample_error">continue</stringProp>
<elementProp name="ThreadGroup.main_controller" elementType="LoopController" guiclass="LoopControlPanel" testclass="LoopController" testname="Loop Controller" enabled="true">
<boolProp name="LoopController.continue_forever">false</boolProp>
<stringProp name="LoopController.loops">2</stringProp>
</elementProp>
<stringProp name="ThreadGroup.num_threads">5</stringProp>
<stringProp name="ThreadGroup.ramp_time">5</stringProp>
<longProp name="ThreadGroup.start_time">1375525852000</longProp>
<longProp name="ThreadGroup.end_time">1375525852000</longProp>
<boolProp name="ThreadGroup.scheduler">false</boolProp>
<stringProp name="ThreadGroup.duration"></stringProp>
<stringProp name="ThreadGroup.delay"></stringProp>
</ThreadGroup>
<hashTree>
<HTTPSamplerProxy guiclass="HttpTestSampleGui" testclass="HTTPSamplerProxy" testname="Rest Request" enabled="true">
<boolProp name="HTTPSampler.postBodyRaw">true</boolProp>
<elementProp name="HTTPsampler.Arguments" elementType="Arguments">
<collectionProp name="Arguments.arguments">
<elementProp name="" elementType="HTTPArgument">
<boolProp name="HTTPArgument.always_encode">false</boolProp>
<stringProp name="Argument.value">{
"name": "new_user",
"job": "user_role"
}</stringProp>
<stringProp name="Argument.metadata">=</stringProp>
</elementProp>
</collectionProp>
</elementProp>
<stringProp name="HTTPSampler.domain">reqres.in</stringProp>
<stringProp name="HTTPSampler.port"></stringProp>
<stringProp name="HTTPSampler.protocol">https</stringProp>
<stringProp name="HTTPSampler.contentEncoding"></stringProp>
<stringProp name="HTTPSampler.path">\api\users</stringProp>
<stringProp name="HTTPSampler.method">POST</stringProp>
<boolProp name="HTTPSampler.follow_redirects">true</boolProp>
<boolProp name="HTTPSampler.auto_redirects">false</boolProp>
<boolProp name="HTTPSampler.use_keepalive">true</boolProp>
<boolProp name="HTTPSampler.DO_MULTIPART_POST">false</boolProp>
<stringProp name="HTTPSampler.embedded_url_re"></stringProp>
<stringProp name="HTTPSampler.connect_timeout"></stringProp>
<stringProp name="HTTPSampler.response_timeout"></stringProp>
</HTTPSamplerProxy>
<hashTree>
<HeaderManager guiclass="HeaderPanel" testclass="HeaderManager" testname="HTTP Header Manager" enabled="true">
<collectionProp name="HeaderManager.headers">
<elementProp name="" elementType="Header">
<stringProp name="Header.name">Content-Type</stringProp>
<stringProp name="Header.value">application/JSON</stringProp>
</elementProp>
</collectionProp>
</HeaderManager>
<hashTree/>
<JSONPathAssertion guiclass="JSONPathAssertionGui" testclass="JSONPathAssertion" testname="JSON Assertion on id" enabled="true">
<stringProp name="JSON_PATH">$.</stringProp>
<stringProp name="EXPECTED_VALUE">[0-9]+</stringProp>
<boolProp name="JSONVALIDATION">true</boolProp>
<boolProp name="EXPECT_NULL">false</boolProp>
<boolProp name="INVERT">false</boolProp>
<boolProp name="ISREGEX">true</boolProp>
</JSONPathAssertion>
<hashTree/>
<JSONPathAssertion guiclass="JSONPathAssertionGui" testclass="JSONPathAssertion" testname="JSON Assertion on createdAt" enabled="false">
<stringProp name="JSON_PATH">$.</stringProp>
<stringProp name="EXPECTED_VALUE">[0-9]{4}-[0-9]{2}-[0-9]{2}T[0-9]{2}:[0-9]{2}:[0-9]{2}\.[0-9]{3}Z</stringProp>
<boolProp name="JSONVALIDATION">true</boolProp>
<boolProp name="EXPECT_NULL">false</boolProp>
<boolProp name="INVERT">false</boolProp>
<boolProp name="ISREGEX">true</boolProp>
</JSONPathAssertion>
<hashTree/>
</hashTree>
</hashTree>
<ResultCollector guiclass="ViewResultsFullVisualizer" testclass="ResultCollector" testname="View Results Tree" enabled="true">
<boolProp name="ResultCollector.error_logging">false</boolProp>
<objProp>
<name>saveConfig</name>
<value class="SampleSaveConfiguration">
<time>true</time>
<latency>true</latency>
<timestamp>true</timestamp>
<success>true</success>
<label>true</label>
<code>true</code>
<message>true</message>
<threadName>true</threadName>
<dataType>false</dataType>
<encoding>false</encoding>
<assertions>true</assertions>
<subresults>false</subresults>
<responseData>false</responseData>
<samplerData>false</samplerData>
<xml>false</xml>
<fieldNames>true</fieldNames>
<responseHeaders>false</responseHeaders>
<requestHeaders>false</requestHeaders>
<responseDataOnError>true</responseDataOnError>
<saveAssertionResultsFailureMessage>true</saveAssertionResultsFailureMessage>
<assertionsResultsToSave>0</assertionsResultsToSave>
<bytes>true</bytes>
<hostname>true</hostname>
<threadCounts>true</threadCounts>
<sampleCount>true</sampleCount>
</value>
</objProp>
<stringProp name="filename"></stringProp>
</ResultCollector>
<hashTree/>
<ResultCollector guiclass="StatGraphVisualizer" testclass="ResultCollector" testname="Aggregate Graph" enabled="true">
<boolProp name="ResultCollector.error_logging">false</boolProp>
<objProp>
<name>saveConfig</name>
<value class="SampleSaveConfiguration">
<time>true</time>
<latency>true</latency>
<timestamp>true</timestamp>
<success>true</success>
<label>true</label>
<code>true</code>
<message>true</message>
<threadName>true</threadName>
<dataType>false</dataType>
<encoding>false</encoding>
<assertions>true</assertions>
<subresults>false</subresults>
<responseData>false</responseData>
<samplerData>false</samplerData>
<xml>false</xml>
<fieldNames>true</fieldNames>
<responseHeaders>false</responseHeaders>
<requestHeaders>false</requestHeaders>
<responseDataOnError>true</responseDataOnError>
<saveAssertionResultsFailureMessage>true</saveAssertionResultsFailureMessage>
<assertionsResultsToSave>0</assertionsResultsToSave>
<bytes>true</bytes>
<hostname>true</hostname>
<threadCounts>true</threadCounts>
<sampleCount>true</sampleCount>
</value>
</objProp>
<stringProp name="filename"></stringProp>
</ResultCollector>
<hashTree/>
</hashTree>
</hashTree>
</jmeterTestPlan>

Answer 1

There are several problems with your script:
You need to use forward slashes in your request path like /api/users
More information: HTML Uniform Resource Locators
Correct mime type for JSON is application/json, for HTTP Headers the names are case-insensitive but the values MAY be case sensitive
More information: HTTP Headers Specification
You need to provide a Json Path expression which will match the value you're trying to assert, to wit $.id instead of $.
More information: API Testing With JMeter and the JSON Extractor

Answer 2

You forgot the key name as createdAt in JSON path expressions
$.createdAt
and id
$.id
You can't have JSON expression as $.