$re = '/^.*?\n(?=Paragraph)/s';
$str = 'Text of variable length spread across several lines Serialization-type text separated by colons (eg ABC:DEF:GHI) A date
From: One line of text To: One or more lines Subject: One or more lines References: One or more lines
From: One line of text To: One
or
more
lines Subject: One or more lines References: One or more lines
From: One line of text To: One or more lines Subject: No references.
From: One line of text To: One or more lines Subject: One
or
more
lines
References: One or more lines
Paragraph 1 Title: A paragraph
Paragraph 2 Title: Another paragraph';
$subst = "";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php