fix(template): treat invalid expression pattern modifier as literal

core/src/main/java/feign/template/Expressions.java

@@ -22,6 +22,7 @@
 import java.util.Optional;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
+import java.util.regex.PatternSyntaxException;
 
 public final class Expressions {
 
@@ -104,15 +105,23 @@ public static Expression create(final String value) {
       }
     }
 
-    /* check for an operator */
-    if (PATH_STYLE_OPERATOR.equalsIgnoreCase(operator)) {
-      return new PathStyleExpression(variableName, variablePattern);
-    }
+    try {
+      /* check for an operator */
+      if (PATH_STYLE_OPERATOR.equalsIgnoreCase(operator)) {
+        return new PathStyleExpression(variableName, variablePattern);
+      }
 
-    /* default to simple */
-    return SimpleExpression.isSimpleExpression(value)
-        ? new SimpleExpression(variableName, variablePattern)
-        : null; // Return null if it can't be validated as a Simple Expression -- Probably a Literal
+      /* default to simple; null if it can't be validated as a Simple Expression -- probably a literal */
+      return SimpleExpression.isSimpleExpression(value)
+          ? new SimpleExpression(variableName, variablePattern)
+          : null;
+    } catch (PatternSyntaxException e) {
+      /*
+       * the ':' value modifier did not contain a valid regular expression, so this is not a usable
+       * expression -- treat it as a literal instead of failing template construction.
+       */
+      return null;
+    }
   }
 
   private static String stripBraces(String expression) {

core/src/test/java/feign/template/ExpressionsTest.java

@@ -44,6 +44,17 @@ void malformedExpression() {
     }
   }
 
+  @Test
+  void invalidPatternModifierIsTreatedAsLiteral() {
+    /*
+     * gh-2739: a header-map value such as "{test:[abc}" parses as a "{name:pattern}" expression,
+     * but the ':' modifier ("[abc") is not a valid regular expression. Compiling it must not blow
+     * up template construction with a PatternSyntaxException -- it should fall back to a literal.
+     */
+    Expression expression = Expressions.create("{test:[abc}");
+    assertThatObject(expression).isNull();
+  }
+
   @Test
   void malformedBodyTemplate() {
     String bodyTemplate = "{" + "a".repeat(65536) + "}";