Reapply "extract regex to reusable Pattern"

This reverts commit 8488e6b.

Author: Picazsoo

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/KotlinClientCodegen.java

@@ -36,6 +36,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.function.Predicate;
+import java.util.regex.Pattern;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
@@ -115,6 +116,7 @@ public class KotlinClientCodegen extends AbstractKotlinCodegen {
     public static final String COMPANION_OBJECT = "companionObject";
 
     protected static final String VENDOR_EXTENSION_BASE_NAME_LITERAL = "x-base-name-literal";
+    private static final Pattern RETROFIT_PATH_PARAM_NAME_PATTERN = Pattern.compile("[a-zA-Z][a-zA-Z0-9_-]*");
 
 
     @Setter protected String dateLibrary = DateLibrary.JAVA8.value;
@@ -1068,7 +1070,7 @@ public OperationsMap postProcessOperationsWithModels(OperationsMap objs, List<Mo
                 if (JVM_RETROFIT2.equals(getLibrary())) {
                     for (CodegenParameter param : operation.allParams) {
                         if (param.isPathParam) {
-                            boolean validRetrofitName = param.baseName.matches("[a-zA-Z][a-zA-Z0-9_-]*");
+                            boolean validRetrofitName = RETROFIT_PATH_PARAM_NAME_PATTERN.matcher(param.baseName).matches();
                             if (validRetrofitName) {
                                 param.vendorExtensions.put("x-retrofit-path-name", param.baseName);
                             } else {