reject assertions in groups, ignore non-capturing groups

When parsing, reject lookahead/behind assertions
(?!...) (?=...) (?<!...) (?<=...)
As well as named capture groups
(?<stuff>...)

Additionally, non-capturing groups are parsed-correctly
(?:...) ==> (...)

src/main/java/dk/brics/automaton/RegExp.java

@@ -722,10 +722,19 @@ private boolean check(int flag) {
 
 	final RegExp parseUnionExp() throws IllegalArgumentException {
 		// check for start anchor, discard if necessary
+		if (peek("^")) {
+			next();
+		}
+
 		RegExp e = parseInterExp();
 		if (match('|'))
 			e = makeUnion(e, parseUnionExp());
 
+		// check for ending anchor, discard if necessary
+		if (peek("$")) {
+			next();
+		}
+
 		return e;
 	}
 
@@ -854,6 +863,19 @@ else if (match('"')) {
 		} else if (match('(')) {
 			if (match(')'))
 				return makeString("");
+			else if (peek("?")) {
+				// figure out if there is group stuff
+				char questionMark = next();
+				if (peek("=!<")) {
+					// =,! -> look ahead
+					// <   -> look behind (<=, <!) or named capture group (<name>)
+					char operator = next();
+					throw new IllegalArgumentException(String.format("group construct %c%c is not supported", questionMark, operator));
+				} else if (peek(":")) {
+					// ?: -> non-capture group
+					next();
+				}
+			}
 			RegExp e = parseUnionExp();
 			if (!match(')'))
 				throw new IllegalArgumentException("expected ')' at position " + pos);

src/test/java/dk/brics/automaton/RegExpTest.java

@@ -80,4 +80,32 @@ public void throwException_when_singleBoundIsTooLarge() {
                 .isInstanceOf(IllegalArgumentException.class)
                 .hasMessage("bound 1000 is too large to be compiled (must be <= 100)");
     }
+
+    @Test
+    public void failToParse_lookAheadAssertion() {
+        assertThatThrownBy(() -> new RegExp("(?![a-z]+)"))
+                .isInstanceOf(IllegalArgumentException.class)
+                .hasMessageContaining("group construct ?!");
+    }
+
+    @Test
+    public void failToParse_lookAheadAssertion2() {
+        assertThatThrownBy(() -> new RegExp("(?=[a-z]+)"))
+                .isInstanceOf(IllegalArgumentException.class)
+                .hasMessageContaining("group construct ?=");
+    }
+
+    @Test
+    public void failToParse_lookBehindAssertion() {
+        assertThatThrownBy(() -> new RegExp("(?<![a-z]+)"))
+                .isInstanceOf(IllegalArgumentException.class)
+                .hasMessageContaining("group construct ?<");
+    }
+
+    @Test
+    public void failToParse_lookBehindAssertion2() {
+        assertThatThrownBy(() -> new RegExp("(?<=[a-z]+)"))
+                .isInstanceOf(IllegalArgumentException.class)
+                .hasMessageContaining("group construct ?<");
+    }
 }