MAINT: Update END statements parsing for recent Fortran standards.

* follow up to gh-18083 covering multi-line uses
of `re.compile(..` and some cases for `re.match(..`
with single (meta)character classes

* replace superfluous single-character regex character
classes with their literal string equivalents; this
avoids the overhead associated with a character class
when there's only a single character enclosed (so there's
no benefit to the class overhead)

* for more information see:
Chapter 6 of:
Friedl, Jeffrey. Mastering Regular Expressions. 3rd ed.,
O’Reilly Media, 2009.

MAINT: exprtype regex simplify

* `determineexprtype_re_1` was modified to remove extraneous
character class markers around a single `,`

* a similar change was applied for the word metacharacter
in `determineexprtype_re_2` and `determineexprtype_re_3`

* the third character class in `determineexprtype_re_3` was
simplified to remove an escape sequence--by placing the
`-` at the start of the character class its metacharacter
status can be avoided