m

Treat string as multiple lines. That is, change "^" and "$"
from matching the start or end of the string to matching 
the start or end of any line anywhere within the string.

s

Treat string as single line. That is, change "." to match 
any character whatsoever, even a newline, which normally it 
would not match.

Used together, as /ms, they let the "." match any character 
whatsoever, while still allowing "^" and "$" to match, 
respectively, just after and just before newlines within 
the string.

x

Extend your pattern's legibility by permitting whitespace 
and comments. Details in /x