<?php
 
$re = '/# Split sentences on whitespace between them.
    (?<=                # Begin positive lookbehind.
      [.!?]             # Either an end of sentence punct,
    | [.!?][\'"]        # or end of sentence punct and quote.
    )                   # End positive lookbehind.
    (?<!                # Begin negative lookbehind.
      Mr\.              # Skip either "Mr."
    | Mrs\.             # or "Mrs.",
    | Ms\.              # or "Ms.",
    | Jr\.              # or "Jr.",
    | Dr\.              # or "Dr.",
    | Prof\.            # or "Prof.",
    | Sr\.              # or "Sr.",
    | T\.V\.A\.         # or "T.V.A.",
                        # or... (you get the idea).
    )                   # End negative lookbehind.
    (?:\h+|^$)          # Split on whitespace between sentences\/empty lines.
    /mix';
 
$text = <<<EOL
This is paragraph one. This is sentence one. Sentence two!
 
This is paragraph two. This is sentence three. Sentence four!
EOL;
 
echo "\nBefore: \n" . $text . "\n";
 
$sentences = preg_split($re, $text, -1);
 
$sentences[1] = " "; // remove 'sentence one'
 
// put text back together
$text = implode( $sentences );
 
echo "\nAfter: \n" . $text . "\n";

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