Line Break Characters
While support for the dot is universal among regex flavors, there are significant differences in which characters they treat as line break characters. All flavors treat the newline \n as a line break. UNIX text files terminate lines with a single newline. All the scripting languages discussed in this tutorial do not treat any other characters as line breaks. This isn't a problem even on Windows where text files normally break lines with a \r\n pair. That's because these scripting languages read and write files in text mode by default. When running on Windows, \r\npairs are automatically converted into \n when a file is read, and \n is automatically written to file as \r\n.
.NET is notably absent from the list of flavors that treat characters other than \n as line breaks. Unlike scripting languages that have their roots in the UNIX world, .NET is a Windows development framework that does not automatically strip carriage return characters from text files that it reads. If you read a Windows text file as a whole into a string, it will contain carriage returns. If you use the regex abc.* on that string, without setting RegexOptions.SingleLine, then it will match abc plus all characters that follow on the same line, plus the carriage return at the end of the line, but without the newline after that.
Some flavors allow you to control which characters should be treated as line breaks. Java has the UNIX_LINES option which makes it treat only \n as a line break. PCRE has options that allow you to choose between \n only, \ronly, \r\n, or all Unicode line breaks.