$re = '/(?xsm) # free-spacing mode, multi-line
(?=.*?pig) # fail right away if pig isn\'t there
(?=
( # Group 1
(?: # skip one line that doesn\'t have pig
^ # beginning of line
(?:(?!pig)[^\r\n])* # zero or more chars not followed by pig
(?:\r?\n) # newline chars
)
(?:(?1)|[^:]+) # recurse Group 1 OR match all chars that are not a :
(:\d+) # match a sequence of digits
)? # End Group 1
) # End lookahead. Group 1, if set, now contains the number of lines skipped
.*?\Kpig # match pig
(?=.*?(?(2)\2):(\d+))/';
$str = 'my cat pi g
dog pi g
my pig
my cow
my mouse
:1:2:3:4:5:6:7';
$subst = "\3";
$result = preg_replace($re, $subst, $str, 1);
echo "The result of the substitution is ".$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