Remove code related to #4239.

Author: mbutrovich

.github/workflows/pr_build_linux.yml

@@ -389,7 +389,6 @@ jobs:
               org.apache.comet.expressions.conditional.CometCaseWhenSuite
               org.apache.comet.CometCodegenDispatchSmokeSuite
               org.apache.comet.CometCodegenSourceSuite
-              org.apache.comet.CometRegExpJvmSuite
           - name: "sql"
             value: |
               org.apache.spark.sql.CometToPrettyStringSuite

.github/workflows/pr_build_macos.yml

@@ -236,7 +236,6 @@ jobs:
               org.apache.comet.expressions.conditional.CometCaseWhenSuite
               org.apache.comet.CometCodegenDispatchSmokeSuite
               org.apache.comet.CometCodegenSourceSuite
-              org.apache.comet.CometRegExpJvmSuite
           - name: "sql"
             value: |
               org.apache.spark.sql.CometToPrettyStringSuite

docs/source/user-guide/latest/compatibility/regex.md

@@ -0,0 +1,24 @@
+<!---
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements.  See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership.  The ASF licenses this file
+to you under the Apache License, Version 2.0 (the
+"License"); you may not use this file except in compliance
+with the License.  You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing,
+software distributed under the License is distributed on an
+"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations
+under the License.
+-->
+
+# Regular Expressions
+
+Comet uses the Rust regexp crate for evaluating regular expressions, and this has different behavior from Java's
+regular expression engine. Comet will fall back to Spark for patterns that are known to produce different results, but
+this can be overridden by setting `spark.comet.expression.regexp.allowIncompatible=true`.