Fix lookaheads with quantifiers

On success, the subpatterns in lookaheads like (?=.*e) had a save
point that persisted, causing the logic in the lookahead group to
be invalid.

Author: natecook1000

Sources/_StringProcessing/ByteCodeGen.swift

@@ -306,7 +306,7 @@ fileprivate extension Compiler.ByteCodeGen {
       save(restoringAt: success)
       save(restoringAt: intercept)
       <sub-pattern>    // failure restores at intercept
-      clearSavePoint   // remove intercept
+      clearThrough(intercept) // remove intercept and any leftovers from <sub-pattern>
       <if negative>:
         clearSavePoint // remove success
       fail             // positive->success, negative propagates
@@ -324,7 +324,7 @@ fileprivate extension Compiler.ByteCodeGen {
     builder.buildSave(success)
     builder.buildSave(intercept)
     try emitNode(child)
-    builder.buildClear()
+    builder.buildClearThrough(intercept)
     if !positive {
       builder.buildClear()
     }

Tests/RegexTests/MatchTests.swift

@@ -891,8 +891,7 @@ extension RegexTests {
       input: "Price: 100 dollars", match: nil)
     firstMatchTest(
       #"(?=\d+ dollars)\d+"#,
-      input: "Price: 100 dollars", match: "100",
-      xfail: true) // TODO
+      input: "Price: 100 dollars", match: "100")
 
     firstMatchTest(
       #"\d+(*pla: dollars)"#,
@@ -917,6 +916,14 @@ extension RegexTests {
       #"\d+(*negative_lookahead: dollars)"#,
       input: "Price: 100 pesos", match: "100")
 
+    // More complex lookaheads
+    firstMatchTest(
+      #"(?=.*e)(?=.*o)(?!.*z)"#,
+      input: "hello", match: "")
+    firstMatchTest(
+      #"^(?=.*e)(?=.*o)(?!.*h)"#,
+      input: "hello", match: nil)
+
     firstMatchTest(
       #"(?<=USD)\d+"#, input: "Price: USD100", match: "100", xfail: true)
     firstMatchTest(