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Asked: 2020-04-29 04:14:19 +0800 CST 2020-04-29 04:14:19 +0800 CST

Use groups to validate text that does not follow the XYYX pattern

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I'm learning RegEx, and an example from: Regex Golf (see level 6, "Abba") caught my attention.

The proposed problem is to filter all the words that do not contain letters that follow the pattern XYYX.

For example, leaving out:

  • abba
  • an alla gmatic
  • b assa risk
  • chorio alla ntois
  • c occo myces


One of the suggested answers was

^(?!.*(.)(.)\2\1)

Regular expression visualization

Debugx Demo


Here is my interpretation of the expression:

  1. Start of the line^
  2. If not followed by 0 or + characters.*
  3. Group the next character in 1(.)
  4. Group the next character in 2(.)
  5. Group 2\2
  6. Group 1\1

The problem is that, even though I'm starting to write my first regular expressions and have already done some exercises, this particular problem leaves me with a lot of questions.

Can anyone give a more intuitive interpretation?

regex

1 Answers

  1. Best Answer

    Here's another explanation:

    • ^, string start
    • (?!, look ahead to see if there is not the following
    •    .*, any character (except \n), as many as possible
    •    (.), any character (except \n), 1st group
    •    (.), any character (except \n), 2nd group
    •    \2, the same as the 2nd group
    •    \1, the same as the 1st group
    • )end of looking forward

    What this regular expression does is search the string for a text that is not followed (from the beginning of the string) by the pattern .*(.)(.)\2\1.

    For example, for the string anallagmatic, the pattern .*(.)(.)\2\1holds in . However, since you don't want to find such a pattern in the string, then the entire regular expression, , does not hold.anallagmatic^(?!.*(.)(.)\2\1)

    Depending on the implementation of the regular expression engine 1 , it will search for the pattern, based on the regular expression ( regex-directed ) or on the text ( text-directed ), in the string and report whether it was found or not.

    For example, the site's Regex Debuggerhttps://regex101.com/ shows the steps taken by the engine until it "failed" (no matches found) with the text anallagmatic:

    Grades

    1. See First Look at How a Regex Engine Works Internally .
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