[Support][NFC] Add test documenting that empty Regex pattern matches nothing.

[martinboehme]

I was wondering about this when I recently used Regex, and I thought it would
be nice to have a test documenting this behavior.

[llvmbot]

@llvm/pr-subscribers-llvm-support

Author: None (martinboehme)

Changes

I was wondering about this when I recently used Regex, and I thought it would
be nice to have a test documenting this behavior.

---

Full diff: https://github.com/llvm/llvm-project/pull/83849.diff

1 Files Affected:

• (modified) llvm/unittests/Support/RegexTest.cpp (+8)

diff --git a/llvm/unittests/Support/RegexTest.cpp b/llvm/unittests/Support/RegexTest.cpp
index 09f674bb209c07..c6ac42591d1fc7 100644
--- a/llvm/unittests/Support/RegexTest.cpp
+++ b/llvm/unittests/Support/RegexTest.cpp
@@ -60,6 +60,14 @@ TEST_F(RegexTest, Basics) {
   EXPECT_TRUE(r5.match(String));
 }
 
+TEST_F(RegexTest, EmptyPattern) {
+  // The empty pattern doesn't match anything -- not even the empty string.
+  // (This is different from some other regex implementations.)
+  Regex r("");
+  EXPECT_FALSE(r.match("123"));
+  EXPECT_FALSE(r.match(""));
+}
+
 TEST_F(RegexTest, Backreferences) {
   Regex r1("([a-z]+)_\\1");
   SmallVector<StringRef, 4> Matches;

[dwblaikie]

if this is different from other regex implementations, could we make it do something more like other implementations so as to be less surprising? (is this a feature of the underlying regex library we use - or something we added on top?)

Couldn't find clear documentation in the libc regex (at a glance/quick googling) that this is all based on - but doesn't look like we special case it in any way, so probably a feature of libc's regex...

[martinboehme]

CI failure is a failing test in Flang that is unrelated.

[martinboehme]

if this is different from other regex implementations, could we make it do something more like other implementations so as to be less surprising? (is this a feature of the underlying regex library we use - or something we added on top?)

I don't see anything in Regex.cpp that special-cases this, so I assume it's a feature of the libc regex code.

Couldn't find clear documentation in the libc regex (at a glance/quick googling) that this is all based on - but doesn't look like we special case it in any way, so probably a feature of libc's regex...

Agree.

I have to admit I'm not sure why we do this, or why the libc regex does it -- but I have to assume that some user of Regex relies on this behavior (knowingly or unknowingly), so I'm hesitant to change it. I did, however, at least want to get a test in documenting the behavior.