Add Regex::CompileOptions::MULTILINE_ONLY

spec/std/regex_spec.cr

@@ -250,6 +250,13 @@ describe "Regex" do
         end
       end
 
+      describe "multiline_only" do
+        it "anchor" do
+          ((/^foo.*$/m).match("foo\nbar")).try(&.[](0)).should eq "foo\nbar"
+          ((Regex.new("^foo.*?", Regex::Options::MULTILINE_ONLY)).match("foo\nbar")).try(&.[](0)).should eq "foo"
+        end
+      end
+
       describe "extended" do
         it "ignores white space" do
           /foo   bar/.matches?("foobar").should be_false

src/regex.cr

@@ -240,12 +240,17 @@ class Regex
     # flag that activates both behaviours, so here we do the same by
     # mapping `MULTILINE` to `PCRE_MULTILINE | PCRE_DOTALL`.
     # The same applies for PCRE2 except that the native values are 0x200 and 0x400.
+    #
+    # For the behaviour of `PCRE_MULTILINE` use `MULTILINE_ONLY`.
 
     # Multiline matching.
     #
     # Equivalent to `MULTILINE | DOTALL` in PCRE and PCRE2.
     MULTILINE = 0x0000_0006
 
+    # Equivalent to `MULTILINE` in PCRE and PCRE2.
+    MULTILINE_ONLY = 0x0000_0004
+
     DOTALL = 0x0000_0002
 
     # Ignore white space and `#` comments.

src/regex/pcre.cr

@@ -36,7 +36,8 @@ module Regex::PCRE
       if options.includes?(option)
         flag |= case option
                 when .ignore_case?       then LibPCRE::CASELESS
-                when .multiline?         then LibPCRE::DOTALL | LibPCRE::MULTILINE
+                when .multiline?         then LibPCRE::MULTILINE | LibPCRE::DOTALL
+                when .multiline_only?    then LibPCRE::MULTILINE
                 when .dotall?            then LibPCRE::DOTALL
                 when .extended?          then LibPCRE::EXTENDED
                 when .anchored?          then LibPCRE::ANCHORED

src/regex/pcre2.cr

@@ -67,7 +67,8 @@ module Regex::PCRE2
       if options.includes?(option)
         flag |= case option
                 when .ignore_case?       then LibPCRE2::CASELESS
-                when .multiline?         then LibPCRE2::DOTALL | LibPCRE2::MULTILINE
+                when .multiline?         then LibPCRE2::MULTILINE | LibPCRE2::DOTALL
+                when .multiline_only?    then LibPCRE2::MULTILINE
                 when .dotall?            then LibPCRE2::DOTALL
                 when .extended?          then LibPCRE2::EXTENDED
                 when .anchored?          then LibPCRE2::ANCHORED