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Don’t Use Regular Expressions To Parse IP Addresses!

Posted by jpluimers on 2017/09/21

Interesting piece: Don’t Use Regular Expressions To Parse IP Addresses! [WayBack]

TL;DR:

When have neither then for quad-dotted decimal IPv4 addresses (ignoring for instance octals and grouped quads), this is suitable: regex – Regular expression to match DNS hostname or IP Address? – Stack Overflow [WayBack]

ValidIpAddressRegex = "^(([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.){3}([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])$";

Which explained looks like this:

https://regex101.com/r/Wyr2Zd/1

Regular expression:

/ ^(([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.){3}([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])$ / g

Explanation:

  • ^ asserts position at start of the string
    • 1st Capturing Group (([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.){3}
      • {3} Quantifier — Matches exactly 3 times
        A repeated capturing group will only capture the last iteration. Put a capturing group around the repeated group to capture all iterations or use a non-capturing group instead if you’re not interested in the data

        • 2nd Capturing Group ([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])
          • 1st Alternative [0-9]
            • Match a single character present in the list below [0-9]
              0-9 a single character in the range between 0 (ASCII 48) and 9 (ASCII 57) (case sensitive)
          • 2nd Alternative [1-9][0-9]
            • Match a single character present in the list below [1-9]
              1-9 a single character in the range between 1 (ASCII 49) and 9 (ASCII 57) (case sensitive)
            • Match a single character present in the list below [0-9]
              0-9 a single character in the range between 0 (ASCII 48) and 9 (ASCII 57) (case sensitive)
          • 3rd Alternative 1[0-9]{2}
            • 1 matches the character 1 literally (case sensitive)
            • Match a single character present in the list below [0-9]{2}
              {2} Quantifier — Matches exactly 2 times
              0-9 a single character in the range between 0 (ASCII 48) and 9 (ASCII 57) (case sensitive)
          • 4th Alternative 2[0-4][0-9]
            • 2 matches the character 2 literally (case sensitive)
            • Match a single character present in the list below [0-4]
              0-4 a single character in the range between 0 (ASCII 48) and 4 (ASCII 52) (case sensitive)
            • Match a single character present in the list below [0-9]
              0-9 a single character in the range between 0 (ASCII 48) and 9 (ASCII 57) (case sensitive)
          • 5th Alternative 25[0-5]
            • 25 matches the characters 25 literally (case sensitive)
            • Match a single character present in the list below [0-5]
              0-5 a single character in the range between 0 (ASCII 48) and 5 (ASCII 53) (case sensitive)
        • \. matches the character . literally (case sensitive)
    • 3rd Capturing Group ([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])
      • 1st Alternative [0-9]
        • Match a single character present in the list below [0-9]
          0-9 a single character in the range between 0 (ASCII 48) and 9 (ASCII 57) (case sensitive)
      • 2nd Alternative [1-9][0-9]
        • Match a single character present in the list below [1-9]
          1-9 a single character in the range between 1 (ASCII 49) and 9 (ASCII 57) (case sensitive)
        • Match a single character present in the list below [0-9]
          0-9 a single character in the range between 0 (ASCII 48) and 9 (ASCII 57) (case sensitive)
      • 3rd Alternative 1[0-9]{2}
        • 1 matches the character 1 literally (case sensitive)
        • Match a single character present in the list below [0-9]{2}
          {2} Quantifier — Matches exactly 2 times
          0-9 a single character in the range between 0 (ASCII 48) and 9 (ASCII 57) (case sensitive)
      • 4th Alternative 2[0-4][0-9]
        • 2 matches the character 2 literally (case sensitive)
        • Match a single character present in the list below [0-4]
          0-4 a single character in the range between 0 (ASCII 48) and 4 (ASCII 52) (case sensitive)
        • Match a single character present in the list below [0-9]
          0-9 a single character in the range between 0 (ASCII 48) and 9 (ASCII 57) (case sensitive)
      • 5th Alternative 25[0-5]
        • 25 matches the characters 25 literally (case sensitive)
        • Match a single character present in the list below [0-5]
          0-5 a single character in the range between 0 (ASCII 48) and 5 (ASCII 53) (case sensitive)
  • $ asserts position at the end of the string, or before the line terminator right at the end of the string (if any)
  • Global pattern flags
    g modifier: global. All matches (don’t return after first match)

–jeroen

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