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Comment by gnosis | Hacker News Reader

  ack '(?=silver).*needle' haystack

is really not the same as:

  grep needle haystack|grep silver

Because with the double grep method, a match will be made whether "silver" is before or after "needle"; while with the single ack command shown above, a match will only be made when "silver" comes before "needle".

Also, I'm not sure why the author uses

  ack '(?=silver).*needle' haystack

instead of simply:

  ack 'silver.*needle' haystack

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Maybe it's because you can write the latter also with plain grep:

    grep 'silver.*needle' haystack

That assumes that "silver" will appear before "needle", which may not always be the case. `grep needle file | grep silver` gets you lines containing both "silver" and "needle" but not necessarily in that order.

Also, traditional grep can easily find 'silver needle': `grep silver\ needle haystack`.

ack hilights results by default, so `(?=silver.*)needle` would potentially be useful. But, yes, not the same as the grep|grep.

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