$re = '/\n +(?=[[:alpha:]])/m';
$str = 'this line
should be joined with this line
and with this line
but this line has a
0 at the front which is not alphabetic so should not be joined
0 an indented line
but what should happen here?';
$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