re = /(?xsm) # free-spacing mode, DOTALL, multi-line
(?=.*?blue) # if blue isn't there, fail without delay
###### Recursive Section ######
# This section aims to balance empty lines with digits, i.e.
# emptyLine,emptyLine,emptyLine ... ~1~2~3
# The last digit block is captured to Group 2, e.g. ~3
(?= # lookahead
( # Group 1
(?: # skip one line that doesn't contain blue
^ # start of line
(?:(?!blue)[^\r\n])* # zero or more chars not followed by blue
(?:\r?\n) # newline
)
(?:(?1)|[^~]+) # recurse Group 1 OR match all non-tilde chars
(~\d+) # match a sequence of digits
)? # End Group 1
) # End lookahead.
# Group 2, if set, now contains the number of lines skipped
.*? # lazily match chars up to...
blue # match blue
.*? # lazily match chars up to...
(?(2)\2) # if Group 2 is set, match Group 2
~ # Match the next tilde
\K # drop what was matched so far
\d+ # match the next digits: this is the match
/
str = 'Paint it white
Paint it black
Why not blue?
Or red or brown?
~1~2~3~4~5~6~7~8~9~10'
# Print the match result
str.match(re) do |match|
puts match.to_s
end
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