Match the nth word in a line

Question

In the app I use, I cannot select a match Group 1.

The result that I can use is the full match from a regex.

but I need the 5th word "jumps" as a match result and not the complete match "The quick brown fox jumps"

^(?:[^ ]*\ ){4}([^ ]*)

The quick brown fox jumps over the lazy dog

Here is a link https://regex101.com/r/nB9yD9/6

Sphene

What language/tool are you using? Does it support variable length lookbehind? – Kentiggerma

the app is ecoDMS 18.09 the programming language as i know is qt 5.7, i dont know if it supports lookbehind, i can try it, can you show me a sample regex? – Sphene

Have a try with ^(?<=(?:\S+ ){4})\S+ or ^(?:\S+ ){4}\S+ – Kentiggerma

i try this "(?<=(\b\s))(\w*)" and it gives me the second word "quick" but how can i get the third word "brown" and the so on "fox" ... – Sphene

Which word do you need? In the question you've said the 5th but in comment you say the 3rd, could you clarify by editing the question? – Kentiggerma

i need all words from a string with six words, for every word i need a single pattern, the only restriction is, that the app takes only the full match from regex, no group1 group2 etc – Sphene

If I well understand, you want a regex for the first word, another one for the second and so on? 1srt: \w+, 2nd: ^(?<=\w+ )\w+, 3rd: ^(?<=(?:\w+ ){2})\w+, 4th: ^(?<=(?:\w+ ){3})\w+ and so on... Is that what you want? – Kentiggerma

Yes, that is what i need, but if i use something like this "^(?<=(?:\w+ ){2})\w+" i got "? The preceding token is not quantifiable" – Sphene

I guess your app doesn't support variable length lookbehind. Have a try with: ^\w+ \K\w+ and ^(?:\w+ ){2}\K\w+ and so on... – Kentiggerma

\K like this "^(?:\w+ ){2}\K\w+" does not work, without it like this "^(?:\w+ ){2}\K\w+" i got a full match with three words. ( regex101.com/r/pR22LK/1 ) – Sphene

It works regex101.com/r/pR22LK/2 with PCRE. Your app doesn't seem to support it, but I don't know how it works. I think you have to extract all the words in an array then select the ones you want. – Kentiggerma

Hello Toto, your solution works in the the App too, like PCRE, thanks !!! – Sphene

Answer 1

It works https://regex101.com/r/pR22LK/2 with PCRE. Your app doesn't seem to support it, but I don't know how it works. I think you have to extract all the words in an array then select the ones you want. – Toto 23 hours ago

Hello Toto, your solution works in the the App too, like PCRE, thanks !!! – gsxr1300 just now edit

Sphene

Answer 2

Since you need the entire match to be only the n-th word, you can try to use 'positive lookbehind', which allows you to only match something, if it is preceded by something else.

To match only the fifth word, you want to match the first word that has four words before it.

To match four words (i.e. word characters followed by a space character):

(\w+\s){4}

To match a single word, but only if it was preceded by four other words:

(?<=(\w+\s){4})(\w+)

Test the result here https://regex101.com/r/QIPEkm/1

Glaze

? The preceding token is not quantifiable ,Your pattern contains one or more errors, please see the explanation section above. – Sphene

The regex functions just fine for me on regex101, in RegexBuddy and in a simple Python script. I'm assuming you changed something that broke it? – Glaze

Answer 3

To find the 3rd word of sentence, use:

^(?:\w+ ){2}\K\w+

Explanation:

^               # beginning of line
    (?:         # start non capture group
        \w+     # 1 or more word character
                # a space
    ){2}        # group must appear twice (change {2} in {3} to get the 4th word and so on)
    \K          # forget all we have seen until this position
    \w+         # 1 or more word character

Demo

Kentiggerma

Answer 4

To match "the first" four words (i.e. word characters followed by a space character):

^(\w+\s){4}

To match a single word, but only if it was preceded by "the first" four other words:

(?<=^(\w+\s){4})(\w+)

Note the ^ difference

If you want to know what this "?<=" mean, check this: https://mcmap.net/q/25940/-regex-lookahead-lookbehind-and-atomic-groups

Thaxter

Can you edit your answer and explain the difference? What does ?<= do in a regular expression and why is that needed here? – Ditzel