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Re: Using Look-ahead and Look-behind

by joewong (Initiate)
on Nov 12, 2007 at 03:34 UTC ( #650210=note: print w/replies, xml ) Need Help??


in reply to Using Look-ahead and Look-behind

Referring to the paragraph "Matching a pattern that doesn't include another pattern", I wonder why ?: is necessary. It seems to be working for me even without ?:. Please explain. thanks.

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Re^2: Using Look-ahead and Look-behind
by Roy Johnson (Monsignor) on Nov 12, 2007 at 17:54 UTC
    Generally, the decision about using ?: is not about whether it's necessary, but that capturing whatever you're grouping isn't necessary. The ?: modifier makes parentheses not capture, which is somewhat more efficient and might make the task of counting left parentheses less onerous.

    By the way, it's not a lookaround feature.


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[ambrus]: The main loop for unixish systems is in https://metacpan. org/source/ KARASIK/Prima-1. 49/unix/apc_app.c. It is a hand-rolled select loop, but also always spins at least once every 0.2 seconds for some reason.
[ambrus]: marto: no, not that way. if you just want to test for an existing id attribute, then write *[@id] , or you can match a regexp to the value of that attribute like *[@id=~/something/ ]
[marto]: ambrus, thanks I ended up going with '//*[starts- with(@id, "thread_")]...
[ambrus]: Sometimes it's easier to just match elements in ways other than xpath, like directly with perl code.

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