perl

kevinr

Oh god not awk and sed. Not unless you're writing something that'll run within a minute of boot-up or on an especially braindead embedded device.

seph

Though it depends a lot on the details.

Mark

if you can put it together with awk and sed in less time than it took you to post the question and read the responses, then awk and sed are right; if there's more to it... perl. (I'd use python instead, of course, but that's not what you were asking)

kyrandil

if you have the restrictions kevinr points out, use (mini/micro)perl

lakmiseiru

perl, or perl oneliners.

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