CODEWITHSUNDEEP
regexexpression
Oleg_08
Oleg_08

Reputation: 487

Regular expression: how to look ahead/behind through the next 5 Words?

In the sentence "with the electric current density of 7 A the test arrangement" I need to check that within 5 words behind or after 7 A stands the word "density". So I use smth. like this [0..9]+[Blank]+A. The question is how to use the commands "look ahead" example1(?=example2) and "look behind" example1(?<=example) to check not only the next or previous word, but the next or previous 5 words? are there another possibilies to match that?

Thanks in advance

Upvotes: 1

Views: 408

Answers (1)

user52889
user52889

Reputation: 1501

Lookaheads can be performed with (?=...) for positive lookahead, (?!...) for negative lookahead. Lookbehind is (?<...) or (?!<...) However, not all regular expression engines support variable-length lookbehinds, so you may have a problem there.

For the lookahead part, a naïve solution (not accounting for punctuation, words composed of things other than /w, etc.) would be: 

7 A(?=\s+(?:\b\w+\s+){0,4}density)

You'll then want to for EITHER the lookahead OR the lookbehind solution.

However, why not just check for: "density" then 0-4 words then 7 A OR 7 A then 0-4 words then "density" without lookaround? Is there a reason you need a zero-width assertion?

That would be something like: 

(?:density(?:\s+\w+\b){0,4}\s+)(7 A)|(7 A)(?:\s+(?:\b\w+\s+){0,4}density)

Upvotes: 1

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