How to make a look-Behind using quantifiers such as the " d+"?

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I need to match the "test" text, but the string will not always start with a fixed number of characters/digits, it may start with any number of characters, example:

001 test

0002 test

20458 test

How do I get the regexp below to work?

(?<=^\d+)test

That look-Behind (?<=^\d+) does not work, gives invalid expression, does not let use the quantifier d+ inside the look-Behind, if I do (?<= d d d d) it gives match only in the first situation. I wish it could be "variable" the number of digits/characters before the match text.

There is how to create a Behind look using +, * or {1 quantifiers,}?

regex

  • Which language, if I’m not mistaken javascript has no lookbehind;;;

  • In fact, closing operators are invalid in lookbehind.

  • do not think it is JS, in the question itself he states that the lookbehind worked when changing the d+

  • JAVA / C#language, in javascript I am aware that there is no lookbehind.

3 answers

6

Instead of using lookbehind, you are best served using a capture group to get a reference to the sequence test:

/^\d+(test)/

Then you access group 1 to get the string test. All right it’s kind of pointless, being a string fixed, but the concept is the same when one wants to extract part of a string of characters.

4


The Response of @Wtrmute this in accordance with your need, but let us focus on your doubt.

How to make a look-Behind using quantifiers?

Answer

You don’t do, the look-Behind would have the logic that you know what comes before of his real capture.
And a quantifier breaks this rule, because if you use a quantifier it is precisely because you do not know how many times "something" should occur.

Addendum

You must remember that the look-Behind sure works backwards.

He will first look for teste after he "walks back" checking the look-Behind.

0002 teste
   ^^|---| captura
   ||- 1ª verificação do look-behind
   |- 2ª verificação do look-behind

  • I understand, well, I will mark this answer as accepted, since unfortunately there is no way to create a regex with dynamic look-Behind with quantifiers and what I want to do is not possible.

2

Regex without lookbehind (remembering that this way you need to access the group1 to get only "test".)

\d*? (teste)

Using lookbehind (this lookbehind identifies the use of 1 digit and space before the word "test", thus capturing independent of the number of digits.)

(?<=\d\s)teste

  • I think I was unhappy in formulating the question, I could not explain properly what I needed, in my example I have only digits before the "test" string, however, there are occurrences of any character, and sometimes whole sentences before my final goal, but I’ve been reading and I think there’s really no way to use a quantifier in the look.

  • It depends on the regex’s Flavour, where are you using it? Can I adjust my answer to suit your need

  • @Thiagohencke . s(test) solves? o . means any character that is not a line break, s indicates space before testing, then () captures what is between it if it matches

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