In my Camel the following appears on p 155, as an example of repeated global subsitution: 
1 while s/(\d)(\d\d\d)(?!\d)/$1,$2)/;

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I'm struggling to understand this. The purpose of the regex is to place commas before every third digit (counting backwards) - i.e. to format them nicely. And indeed, it does do this! But how?

At first I thought it might be a typo. If ! were a : 
1 while s/(\d)(\d\d\d)(?:\d)/$1,$2)/;

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then I'd understand what was going on - we're just saving time by not capturing the third cluster. But then the regex doesn't work. Another way to do it is 
1 while s/(\d)(\d\d\d)(?:,|$)/$1,$2/;

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Which does work, and in a way I understand. But what I still don't get is this ?! business. I can't find anything in the Camel to explain it. Is it negation? But then why? And why doesn't 
1 while s/(\d)(\d\d\d)(?:\D)/$1,$2/;

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work?

I'd be very interested to know what's going on...

§ George Sherston

Edit: Changed title, on author's request. larsen

In reply to ?! idiom in regex - que? by George_Sherston

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