const regex = /(?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+))/;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('(?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+))', '')
const str = `my cat pi g
dog pi g
my pig
my cow
my mouse
:1:2:3:4:5:6:7`;
const subst = `\3`;
// The substituted value will be contained in the result variable
const result = str.replace(regex, subst);
console.log('Substitution result: ', 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 JavaScript, please visit: https://developer.mozilla.org/en/docs/Web/JavaScript/Guide/Regular_Expressions