[...] regolari avanzate con asserzioni negative che “guardano avanti”, proponendo anche un link di approfondimento. In realtà penso che nella maggior parte dei casi si possa ricorrere ad espressioni meno avanzate [...]

[...] can avoid problems with a step matching subsequent steps by using regular expression that have negative lookaheads to exclude the later [...]

Robbin

ps I really don’t get credit here. Steve gets the credit.

This is really powerful Robbin. Everytime I think that RegEx has no more secrets for me, you come up with something new on this blog.

My fellow metro commuters (Serge, Jean-Pierre and Jean-François – just kidding) and myself were just debating about the last example you used: member(?=s)hip, and though it is understood that it will NOT match “membership”, we were wondering what it WOULD match, if anything…
Personally, my money was on it matching “memberhips” (match “member” – as long as it is followed at some point further down the line by an ‘s’ – followed by “hip”). Alas, I’m wrong again, as is so often the case in these situations. I tested it in the RegEx Coach, and it failed miserably. As the RegEx is written, “member” needs to be immediately followed by “s”.

However, I played around with it a bit further and found a way to make these lookaheads function in a “anywhere further down the line” kinda way. If we use a RegEx like the following:

member(?=.*s)hip

then it WILL match “memberhips” (or rather it will match “memberhip” so long as it is followed further in the string by an ‘s’).
I really like the fact that the (?=.*s) leapfrogs the following element “hip” as it illustrates well what you were saying about it being a mere condition that qualifies “member” and NOT an element in the string that needs to be taken into account in the sequence.

We could imagine the follwing URIs:
/checkout.php?…6billion_parameters…&finalOutcome=credit_card_visa_ok&step=purchase
/checkout.php?…6billion_parameters…&finalOutcome=mail_cheque_pending&step=purchase
/checkout.php?…6billion_parameters…&finalOutcome=debit_card_mastercard&step=purchase
/checkout.php?…6billion_parameters…&finalOutcome=credit_card_amex&step=purchase
/checkout.php?…6billion_parameters…&finalOutcome=credit_card_error&step=purchase
etc.

In this instance let us imagine wanting to match all /checkout.php URIs with the parameter step=purchase, UNLESS the value of the “finalOutcome=” parameter is “credit_card_error”.
We could simply use the following RegEx:

/checkout\.php\?(?!.*finalOutcome=credit_card_error).*&step=purchase

Read this as: match all URIs that contain “/checkout.php?” (so long as you can’t find anywhere in the string following “finalOutcome=credit_card_error”), followed by any text, followed by “&step=purchase”

I can’t imagine the number of times I’ve had a need for these conditional lookaheads, but not being aware of their existence, I ended up with a ludicrously verbose and obscene RegEx instead
.Thanks for furthering our knowledge once again

Alan