in reply to Regexp for word with underscore in the middle

Your code should have found do_add. This finds do_add or do_delete. Note that case is important. Make the regex /do_(add|delete)/i (note the i) to make it case insensitive.

use warnings;
use strict;

while (<DATA>) {
   if (/do_(add|delete)/) {
      print "Found: $_";
   }
}

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__DATA__
A line of stuff with do_add in it.
A line of stuff with do_delete in it.
Another line of stuff.
The last line of stuff. It has do_add and do_delete.


Found: A line of stuff with do_add in it.
Found: A line of stuff with do_delete in it.
Found: The last line of stuff. It has do_add and do_delete.

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Perl is Huffman encoded by design.

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