The Ruby Way: Solutions and Techniques in Ruby Programming, Second Edition

3.10. Matching a Newline with a Dot

Ordinarily a dot matches any character except a newline. When the m (multiline) modifier is used, a newline will be matched by a dot. The same is true when the Regexp::MULTILINE option is used in creating a regex:

str = "Rubies are red\nAnd violets are blue.\n"
pat1 = /red./
pat2 = /red./m

str =~ pat1       # nil
str =~ pat2       # 11

This multiline mode has no effect on where anchors match (such as ^, $, \A, and \Z); they match in the same places. All that is affected is whether a dot matches a newline.