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 = str.sub(re, subst)
# Print the result of the substitution
puts 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 Ruby, please visit: http://ruby-doc.org/core-2.2.0/Regexp.html