Revert "SERVER-30986 Prevent invalid regex in RegexMatchExpressions"

There exist PCRE regexes whose error strings are nonempty but can still
be used for matching. This restores the functionality of the (*UCP)
option for SERVER-34933.

This reverts commit 1531b7b7280dd37d5f7ffd49171a65305ad442ba.
Conflicts:
	src/mongo/db/matcher/expression_leaf.cpp
	src/mongo/db/matcher/expression_leaf.h
	src/mongo/db/matcher/expression_leaf_test.cpp

3 files changed, 11 insertions, 10 deletions

diff --git a/src/mongo/db/matcher/expression_leaf.cpp b/src/mongo/db/matcher/expression_leaf.cpp
index c2485fa128a..1f6a4df3f7d 100644
--- a/src/mongo/db/matcher/expression_leaf.cpp
+++ b/src/mongo/db/matcher/expression_leaf.cpp
@@ -196,6 +196,8 @@ inline pcrecpp::RE_Options flags2options(const char* flags) {
     return options;
 }
 
+constexpr size_t RegexMatchExpression::kMaxPatternSize;
+
 RegexMatchExpression::RegexMatchExpression(StringData path, const BSONElement& e)
     : LeafMatchExpression(REGEX, path),
       _regex(e.regex()),
@@ -214,6 +216,9 @@ RegexMatchExpression::RegexMatchExpression(StringData path, StringData regex, St
 }
 
 void RegexMatchExpression::_init() {
+    uassert(
+        ErrorCodes::BadValue, "Regular expression is too long", _regex.size() <= kMaxPatternSize);
+
     uassert(ErrorCodes::BadValue,
             "Regular expression cannot contain an embedded null byte",
             _regex.find('\0') == std::string::npos);
@@ -221,10 +226,6 @@ void RegexMatchExpression::_init() {
     uassert(ErrorCodes::BadValue,
             "Regular expression options string cannot contain an embedded null byte",
             _flags.find('\0') == std::string::npos);
-
-    uassert(ErrorCodes::BadValue,
-            str::stream() << "Regular expression is invalid: " << _re->error(),
-            _re->error().empty());
 }
 
 RegexMatchExpression::~RegexMatchExpression() {}
diff --git a/src/mongo/db/matcher/expression_leaf.h b/src/mongo/db/matcher/expression_leaf.h
index 90c2d9cf0e5..2d33adcc883 100644
--- a/src/mongo/db/matcher/expression_leaf.h
+++ b/src/mongo/db/matcher/expression_leaf.h
@@ -281,8 +281,14 @@ public:
 
 class RegexMatchExpression : public LeafMatchExpression {
 public:
+    /**
+     * The maximum length of regex patterns.
+     */
+    static constexpr size_t kMaxPatternSize = 32764;
+
     RegexMatchExpression(StringData path, const BSONElement& e);
     RegexMatchExpression(StringData path, StringData regex, StringData options);
+
     ~RegexMatchExpression();
 
     virtual std::unique_ptr<MatchExpression> shallowClone() const {
diff --git a/src/mongo/db/matcher/expression_leaf_test.cpp b/src/mongo/db/matcher/expression_leaf_test.cpp
index dde4439d517..87ac6861f7f 100644
--- a/src/mongo/db/matcher/expression_leaf_test.cpp
+++ b/src/mongo/db/matcher/expression_leaf_test.cpp
@@ -896,12 +896,6 @@ TEST(RegexMatchExpression, RegexOptionsStringCannotContainEmbeddedNullByte) {
     }
 }
 
-TEST(RegexMatchExpression, RegexCannotBeInvalid) {
-    const auto invalid = "["_sd;
-    ASSERT_THROWS_CODE(
-        RegexMatchExpression regex("path", invalid, ""), AssertionException, ErrorCodes::BadValue);
-}
-
 TEST(ModMatchExpression, MatchesElement) {
     BSONObj match = BSON("a" << 1);
     BSONObj largerMatch = BSON("a" << 4.0);