i don't understand this RE example from the documentation

[John Salerno]

Ok, I've been staring at this and figuring it out for a while. I'm close
to getting it, but I'm confused by the examples:

(?(id/name)yes-pattern|no-pattern)
Will try to match with yes-pattern if the group with given id or name
exists, and with no-pattern if it doesn't. |no-pattern is optional and
can be omitted.

For example, (<)?(\w+@\w+(?: \.\w+)+)(?(1)>) is a poor email matching
pattern, which will match with '<us**@host.com >' as well as
'us**@host.com' , but not with '<us**@host.com '. New in version 2.4.

group(1) is the email address pattern, right? So why does the above RE
match 'us**@host.com' . If the email address exists, does the last part
of the RE: (?(1)>) mean that it has to end with a '>'?

[John Salerno]

John Salerno wrote:

Ok, I've been staring at this and figuring it out for a while. I'm close
to getting it, but I'm confused by the examples:

(?(id/name)yes-pattern|no-pattern)
Will try to match with yes-pattern if the group with given id or name
exists, and with no-pattern if it doesn't. |no-pattern is optional and
can be omitted.

For example, (<)?(\w+@\w+(?: \.\w+)+)(?(1)>) is a poor email matching
pattern, which will match with '<us**@host.com >' as well as
'us**@host.com' , but not with '<us**@host.com '. New in version 2.4.

group(1) is the email address pattern, right? So why does the above RE
match 'us**@host.com' . If the email address exists, does the last part
of the RE: (?(1)>) mean that it has to end with a '>'?

I think I got it. The group(1) is referring to the opening '<', not the
email address. I had seen an earlier example that used group(0), so I
thought maybe the groups were 0-based.

[Ben Cartwright]

John Salerno wrote:

John Salerno wrote:

Ok, I've been staring at this and figuring it out for a while. I'm close
to getting it, but I'm confused by the examples:

(?(id/name)yes-pattern|no-pattern)
Will try to match with yes-pattern if the group with given id or name
exists, and with no-pattern if it doesn't. |no-pattern is optional and
can be omitted.

For example, (<)?(\w+@\w+(?: \.\w+)+)(?(1)>) is a poor email matching
pattern, which will match with '<us**@host.com >' as well as
'us**@host.com' , but not with '<us**@host.com '. New in version 2.4.

group(1) is the email address pattern, right? So why does the above RE
match 'us**@host.com' . If the email address exists, does the last part
of the RE: (?(1)>) mean that it has to end with a '>'?

I think I got it. The group(1) is referring to the opening '<', not the
email address. I had seen an earlier example that used group(0), so I
thought maybe the groups were 0-based.

The groups *are* 0-based. The 0th group is the whole match, e.g.:

import re
m = re.match(r'a(b+ )', 'abbbb')
m.group(0) 'abbbb' m.group(1) 'bbbb'

And for the pattern you were looking at:
m = re.match(r'(<)? (\w+@\w+(?:\.\w +)+)(?(1)>)', '<fo*@test.com> ')
m.group(0) '<fo*@test.com> ' m.group(1) '<' m.group(2)

'fo*@test.com'

--Ben

[John Salerno]

Ben Cartwright wrote:

The groups *are* 0-based. The 0th group is the whole match, e.g.:

Ah, I see! But I also read that if a group number starts with 0, it is
treated as octal. Did I read that wrong, or maybe it doesn't matter
since 0 is 0 in any base system?