It's impossible to create a regex that is MULTILINE but not DOT_ALL

[ralsina]

Bug Report

The expected behaviour when using a MULTILINE regex like #.*$ is that it will match a whole line that begins with #

Because crystal conflates MULTILINE and DOT_ALL, that regex will, when set to MULTILINE, match also any subsequent lines until the end of the file.

Regex.new("#.*$", Regex::Options::MULTILINE).match("# this should be matched\nthis shouldn't")

=> Regex::MatchData("# this should be matched\nthis shouldn't")

To keep backwards compatibility, the behaviour of MULTILINE in crystal should not be changed, but I propose a change that adds a new MULTILINE_ONLY flag that just doesn't set DOT_ALL when creating the underlying PCRE2 object.

[ralsina]

PR: #14870

[oprypin]

Some previous discussion was here: #8062

Indeed it's sad that DOTALL is bunched together with MULTILINE.

/(?m)#.*$/ achieves the desired behavior of just MULTILINE, by the way - confusingly it's totally different from /#.*$/m