# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/(?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+))/\3/;t' <<< "my cat pi g
dog pi g
my pig
my cow
my mouse
:1:2:3:4:5:6:7"
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 SED, please visit: https://www.gnu.org/software/sed/manual/html_node/The-_0022s_0022-Command.html