r"

(?:\s|^) # Start of string or word boundary
( # Capture Group
(?: # Non-Capture Group for X.
(?: # Nested NCG for 0..255
(?:\d){1,2}| # Allow any single digit or any pair of digits or
(?:1\d{2})| # 3 Digits starting with 1 or
(?:2[0-4]\d)| # 3 Digists starting with 20..24 or
(?:25[0-5]) # 250..255
) # this should allow X=0..255 in the NCG
\. # Must be followed by a literal .
){3} # X. Must repeat exactly 3 times
(?: # NCG for final 0..255
(?:\d{1,2})| # Allow any single digit or any pair of digits or
(?:1\d{2})| # 3 Digits starting with 1 or
(?:2[0-4]\d)| # 3 digits starting 20..24 or
(?:25[0-5]) # 250.255
) # End of final 0.255
) # End of capture group
(?:\s|$) # Must be followed by EOS or wspace

"

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