# 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) # lookahead: if pig is not there, fail right away to save the effort
(?: # start counter-line-skipper (lines that don't include pig)
(?: # 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
)
# for each line skipped, let Group 1 match an ever increasing portion of the numbers string at the bottom
(?= # lookahead
[^:]+ # skip all chars that are not colons
( # start Group 1
(?(1)\1) # if Group 1 is set, match Group 1
# (?>\1?) # alternate phrasing for the above
:\d+ # match a colon and some digits
) # end Group 1
) # end lookahead
)*+ # end counter-line-skipper: zero or more times
.*? # match
\K # drop everything we've matched so far
pig # match pig (this is the match!)
(?=[^:]+(?(1)\1):(\d+)) # capture the next number to Group 2/\2/;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