regex101: Verbose Python IP4 Address

Explanation

Pro

Non-Capturing Group(?:\s|^)

1st Alternative\s

\sa Unicode whitespace character

2nd Alternative^

^start of string

Comment: Start of string or word boundary

Group 1( # 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 )

Comment: Capture Group

Non-Capturing 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}

{3}repeats the group contents below exactly 3 times:

Comment: Non-Capture Group for X.

Non-Capturing Group(?: # 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 )

1st Alternative # Nested NCG for 0..255 (?:\d){1,2}

Comment: Nested NCG for 0..255

Non-Capturing Group(?:\d){1,2}

{1,2}repeats the group contents below between 1 and 2 times:

\da Unicode decimal digit

2nd Alternative # Allow any single digit or any pair of digits or (?:1\d{2})

Comment: Allow any single digit or any pair of digits or

Non-Capturing Group(?:1\d{2})

1the literal 1 (49₁₀ / 61₈ / 31₁₆)

\d{2}a Unicode decimal digit, repeated exactly 2 times

3rd Alternative # 3 Digits starting with 1 or (?:2[0-4]\d)

Comment: 3 Digits starting with 1 or

Non-Capturing Group(?:2[0-4]\d)

2the literal 2 (50₁₀ / 62₈ / 32₁₆)

Character Class[0-4]

0-4one character from 0 (48₁₀) to 4 (52₁₀)

\da Unicode decimal digit

4th Alternative # 3 Digists starting with 20..24 or (?:25[0-5]) # 250..255

Comment: 3 Digists starting with 20..24 or

Non-Capturing Group(?:25[0-5])

25the literal text 25

Character Class[0-5]

0-5one character from 0 (48₁₀) to 5 (53₁₀)

Comment: 250..255

Comment: this should allow X=0..255 in the NCG

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

Comment: Must be followed by a literal .

Comment: X. Must repeat exactly 3 times

Non-Capturing Group(?: # 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 )

1st Alternative # NCG for final 0..255 (?:\d{1,2})

Comment: NCG for final 0..255

Non-Capturing Group(?:\d{1,2})

\d{1,2}a Unicode decimal digit, repeated between 1 and 2 times

2nd Alternative # Allow any single digit or any pair of digits or (?:1\d{2})

Comment: Allow any single digit or any pair of digits or

Non-Capturing Group(?:1\d{2})

1the literal 1 (49₁₀ / 61₈ / 31₁₆)

\d{2}a Unicode decimal digit, repeated exactly 2 times

3rd Alternative # 3 Digits starting with 1 or (?:2[0-4]\d)

Comment: 3 Digits starting with 1 or

Non-Capturing Group(?:2[0-4]\d)

2the literal 2 (50₁₀ / 62₈ / 32₁₆)

Character Class[0-4]

0-4one character from 0 (48₁₀) to 4 (52₁₀)

\da Unicode decimal digit

4th Alternative # 3 digits starting 20..24 or (?:25[0-5]) # 250.255

Comment: 3 digits starting 20..24 or

Non-Capturing Group(?:25[0-5])

Comment: 250.255

Comment: End of final 0.255

Comment: End of capture group

Non-Capturing Group(?:\s|$)

Comment: Must be followed by EOS or wspace

Global pattern flags

g modifier: global. Finds all matches instead of stopping after the first

x modifier: extended. Ignores unescaped whitespace and comments outside character classes

Quick Reference

25 tokens

Regular Expression

(?:\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

123.123.123.145 is an IP4 so is 1.2.3.4 and 1.22.33.44 but 256.1.2.3 isn't 254.1.2.3 is but 260.1.1.1 isn't nor 2.5.256.1 and 1.2.3.4:222 IP6 1.2.3.4.5.6