title: Perl regexp: how to start matching text in part of file after I have find a "flag" string in the context?
body: <p>Apologize for the way I describe my question, maybe it will be much clarified if I give the instance as below:
  Consider the case that I have a certain file, and it is separated into different section with each beginning with a certain string consider as flag: e.g.
  consider "From Clock" as the flag I mentioned about
  example_file:</p>
  
  <pre><code>From Clock:  fdbk_bufg_cell_in_net
    To Clock:  fdbk_bufg_cell_in_net
  
  Setup :           NA  Failing Endpoints,  Worst Slack           NA  ,  Total Violation           NA
  Hold  :           NA  Failing Endpoints,  Worst Slack           NA  ,  Total Violation           NA
  PW    :            0  Failing Endpoints,  Worst Slack        5.501ns,  Total Violation        0.000ns
  </code></pre>
  
  <p>Pulse Width Checks</p>
  
  <pre><code>Clock Name:         fdbk_bufg_cell_in_net
  Waveform(ns):       { 0.000 3.500 }
  Period(ns):         7.000
  Sources:            { my_atspeed_mmcm/CLKFBOUT }
  
  Check Type  Corner  Lib Pin   Reference Pin  Required(ns)  Actual(ns)  Slack(ns)  Location      Pin
  Min Period  n/a     BUFGCE/I  n/a            1.499         7.000       5.501      BUFGCE_X0Y35  fdbk_bufg_cell/I
  </code></pre>
  
  <hr>
  
  <pre><code>From Clock:  mmcm_clkout
    To Clock:  mmcm_clkout
  
  Setup :           NA  Failing Endpoints,  Worst Slack           NA  ,  Total Violation           NA
  Hold  :            0  Failing Endpoints,  Worst Slack        0.123ns,  Total Violation        0.000ns
  PW    :          625  Failing Endpoints,  Worst Slack       -0.195ns,  Total Violation     -121.875ns
  </code></pre>
  
  <p>Now I only want to match the "PW :" line in "From Clock: mmcm_clkout" part, how do I do so?</p>