CODEWITHSUNDEEP
regexregex-lookarounds
Adam Shakhabov
Adam Shakhabov

Reputation: 1356

Regex expression to match tab

I need a regular expression that matches tab symbol by the following rules:

"—>text"       does not match
"1.—>text"     does not match
"1—>text"      does not match
"A.—>text"     does not match
"text—>text"   match

That is, it shouldn't match tabs that are at the beginning of the text, after a listed item mark [A-Z] or [0-9]. Here is my expression:

(?<!^((?:\d+|[A-Z])(?:\.)?))\t(?!\1)

https://regex101.com/r/zgJAG9/1

It does not work for all cases:

enter image description here

How to fix it?

Upvotes: 3

Views: 8446

Answers (1)

Wiktor Stribiżew
Wiktor Stribiżew

Reputation: 627126

You can use

(?<!^(?:(?:\d+|[A-Z])\.?)?)\t

See the regex demo. Details:

  • (?<!^(?:(?:\d+|[A-Z])\.?)?) - a negative lookbehind that fails the match if, immediately to the left of the current location, there are
    • ^ - start of string
    • (?:(?:\d+|[A-Z])\.?)? - an optional sequence of
      • (?:\d+|[A-Z]) - one or more digits or an uppercase ASCII letter
      • \.? - an optional .
  • \t - a tab char.

Note that (?:\.)? is the same as \.?.

Also, capturing groups inside a negative lookbehind makes little sense as regex processing will be stopped before your backreference pattern is reached.

Upvotes: 3

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