Can regex support mismatching '\n'?

[freekeeper]

RE2 support does not match newlines. Like this, by setting dot_nl_ = true, the regular expression can not match the \n character.

Does regex have such a function, or in what way can it be achieved?

[BurntSushi]

. does not match \n by default, like just about every other regex engine. And like almost every other regex engine, you can make . match \n by enabling the s flag.

[freekeeper]

There are the following test cases in RE2

static struct NeverTest {
  const char* regexp;
  const char* text;
  const char* match;
} never_tests[] = {
  { "(.*)", "abc\ndef\nghi\n", "abc" },
  { "(?s)(abc.*def)", "abc\ndef\n", NULL },
  { "(abc(.|\n)*def)", "abc\ndef\n", NULL },
  { "(abc[^x]*def)", "abc\ndef\n", NULL },
  { "(abc[^x]*def)", "abczzzdef\ndef\n", "abczzzdef" },
};

TEST(RE2, NeverNewline) {
  RE2::Options opt;
  opt.set_never_nl(true);
  for (size_t i = 0; i < arraysize(never_tests); i++) {
    const NeverTest& t = never_tests[i];
    RE2 re(t.regexp, opt);
    if (t.match == NULL) {
      EXPECT_FALSE(re.PartialMatch(t.text, re));
    } else {
      StringPiece m;
      EXPECT_TRUE(re.PartialMatch(t.text, re, &m));
      EXPECT_EQ(m, t.match);
    }
  }
}

In the above test case, never_tests[1].regexp = "(?s)(abc.*def)", never_tests[1].text = "abc\ndef\n", (?s) means make . match \n.

But by setting opt.set_never_nl(true), all regular expressions cannot match \n. Can this pattern be implemented in regex or in what way?

According to my reading of the regex source code, I found that regex implements the dot_nl mode in RE2. But regex does not seem to implement the never_nl mode in RE2. So I wonder if regex can implement the never_nl mode in RE2.

[BurntSushi]

never_nl and dot_nl are two very different options. There is no out-of-the-box never_nl option in this crate, but you can implement it without too much fuss yourself by using the regex-syntax crate and transforming the Hir. Here's the code ripgrep uses to do that exact thing: https://github.com/BurntSushi/ripgrep/blob/65b1b0e38a973ce15d887a1b01d27cabe391cd24/crates/regex/src/strip.rs

[freekeeper]

Thank you so much!