I agree with what you say regarding the flexibility of the (?i) construct over the global m{...}i one but I think I would take issue with your second paragraph.

... the unvarying use of the /xms regex modifier 'tail' (if that's the proper term) is to give the ^ $ . regex operators unvarying behaviors ...

While they are very rarely used that way, m, s and even x are no more invariant than i and can be sprinkled throughout your regular expression. To give a nonsense example:

knoppix@Microknoppix:~$ perl -E '
> $_ = qq{aabb\nwxy935TXB\n123};
> say $1 if m{(?x) ( a [^a] (?s) .* 9 (?-s) .* ) };'
abb
wxy935TXB
knoppix@Microknoppix:~$

[download]
I'm also under the impression that (?i) need not be confined solely to the scope of capturing and non-capturing groups but can also be used as a "switch" to change the matching behaviour from the point at which it appears onwards or in its own "modifier" group ((?i:pattern)) for want of a better word. The following patterns are examples of how I understand the modifiers can be used:

m{(?i)Whole pattern case-insensitive}
m(Case-Sensitive(?i)case-insensitive(?-i)Case-Sensitive Again}
m{Case-Sensitive((?i)except in this capture)Case-Sensitive Again}
m{Case-Sensitive(?i:but not here)Case-Sensitive Again}
m{all case-insensitive(?-i:Except Here) and insensitive again}i
m{(?x) Use white-space\sfor readability(?-x)but literal spaces now};

[download]

PBP is a fascinating book with very well argued recommendations that make you wonder whether you are doing things the right way. I believe that there are two equally valid reactions to each recommendation in the book: follow the recommendation if, after consideration, it seems better than what you were doing before; alternatively, if you can come up with equally cogent arguments for continuing the way you were, then do that. The main thing is that the book has made you think.

Cheers,

JohnGG

In reply to Re^4: Capturing string matched by regex by johngg
in thread Capturing string matched by regex by tunafish

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