m (PCRE_MULTILINE)
      By default, PCRE treats the subject string as consisting of a
      single "line" of characters (even if it actually contains
      several newlines). The "start of line" metacharacter (^)
      matches only at the start of the string, while the "end of
      line" metacharacter ($) matches only at the end of the
      string, or before a terminating newline (unless
      D modifier is set). This is the same as
      Perl.
     
      When this modifier is set, the "start of line" and "end of
      line" constructs match immediately following or immediately
      before any newline in the subject string, respectively, as
      well as at the very start and end. This is equivalent to
      Perl's /m modifier. If there are no "\n" characters in a
      subject string, or no occurrences of ^ or $ in a pattern,
      setting this modifier has no effect.

ex)

$str = '0123456789\nabcdefg\nABCDEFG';

/pattern/ (normal)

^ => 0
$ => G


/pattern/m (multi-line "\n")

^ => 0, a, A
$ => 9, g, G