bug#10885: Replace expressions: enhance functionality when searching in filled paragraphs

[Juri Linkov <juri@jurta.org>]

> 1.  You've defined two separate variables (`isearch-lax-whitespace'
> and `isearch-regexp-lax-whitespace') to enable/disable the lax
> whitespace matching in search commands: one for basic search commands
> and the other for regexp search commands.  But there is only one
> similar variable (replace-lax-whitespace) which controls both basic
> and regexp replace commands.  Why this inconsistency?  I.e. why not
> define also a `replace-regexp-lax-whitespace' variable?

`isearch-regexp-lax-whitespace' was necessary to provide
backward-compatibility for old functionality.  Very likely
it will be declared obsolete.  But of course, it would be better
to have `replace-regexp-lax-whitespace' for consistency until
they both will be declared obsolete simultaneously.

> 2.  While in an incremental search commands, it is possible to toggle
> the value of the corresponding variable with `M-s SPC'.  Why not
> having the same possibility in incremental replace commands?

Isearch has different implementation than query-replace.
query-replace uses the normal minibuffer to read a string to replace.
Implementing `M-s SPC' for it means more trouble:
`query-replace-read-from' should set the arg `keymap' of
`read-from-minibuffer' to a new keymap with the `M-s SPC' keybinding
bound to a function to toggle the value of the defcustom option.

> 3.  Many users will want a consistent behavior wrt whitespace-matching
> between (regexp) search and (regexp) replace commands.  So, why not
> allowing to "connect" the corresponding variables?  I.e. why not
> defining some special value for `replace-lax-whitespace' and
> `replace-regexp-lax-whitespace' which means "get the value from the
> corresponding search variable" ?

When it will be decided that isearch and query-replace should have
the same default values, then `replace-lax-whitespace' could inherit
its default value from `isearch-lax-whitespace'.