by default, the dot can match any character except newline character, \n.
yet, in many language, there is an option to enable dot match \n. in Perl, this can be done by using single-mode, which is turned on using /s
“In Perl, the mode where the dot also matches newlines is called “single-line mode”. This is a bit unfortunate, because it is easy to mix up this term with “multi-line mode”. Multi-line mode only affects anchors, and single-line mode only affects the dot. You can activate single-line mode by adding an s after the regex code, like this: m/^regex$/s;. ”
Note, single-mode and multi-mode here doesn’t conflict with each other as explained in above cited sentenc
about mutli-line mode:
If you have a string consisting of multiple lines, like first line\nsecond line (where \n indicates a line break), it is often desirable to work with lines, rather than the entire string. Therefore, all the regex engines discussed in this tutorial have the option to expand the meaning of both anchors. ^ can then match at the start of the string (before the f in the above string), as well as after each line break (between \n and s). Likewise, $ will still match at the end of the string (after the last e), and also before every line break (between e and \n).
In text editors like EditPad Pro or GNU Emacs, and regex tools like PowerGREP, the caret and dollar always match at the start and end of each line. This makes sense because those applications are designed to work with entire files, rather than short strings.
In all programming languages and libraries discussed on this website , except Ruby, you have to explicitly activate this extended functionality. It is traditionally called “multi-line mode”. In Perl, you do this by adding an m after the regex code, like this: m/^regex$/m;. In .NET, the anchors match before and after newlines when you specify RegexOptions.Multiline, such as in Regex.Match("string", "regex", RegexOptions.Multiline).