use Perl

• You keep matching against $str (Score:1)

  by Whammo (2555)

  You reset $str2 (in essence) on every loop, but you continually match against the same $str.
  
  So if you unroll your loops, you get basically this.
  
  Look for, at the beginning of the string, "J:", followed by a bunch of stuff that isn't a newline. (.+ with /m). Did you succeed? Remember this spot and proceed.
  
  Look for, at the beginning of the string (where we left off), "I:", followed by a bunch of stuff that isn't a newline. Did you succeed? *Remember this spot and proceed*.
  
  Look for, at the beginning of
• From perlop (Score:1)

  by runrig (3385)

  What Whammo [perl.org] said, and from perlop:

  In scalar context, each execution of "m//g" finds the next match, returning true if it matches, and false if there is no further match. The position after the last match can be read or set using the pos() function; see the pos entry in the perlfunc manpage. A failed match normally resets the search position to the beginning of the string, but you can avoid that by adding the "/c" modifier (e.g. "m//gc"). Modifying the target string also resets the search position.