"..\A\B\a.blah"

"../A/B/a.blah"
"..//A//B//a.blah"
"..ABa.blah"

"..\A\B\a.blah"

ffap-string-at-point-mode-alist is a variable defined in ‘ffap.el’.
Its value is ((file "--:\\${}+<>@-Z_[:alpha:]~*?" "<@" "@>;.,!:")
 (url "--:=&?$+@-Z_[:alpha:]~#,%;*()!'" "^[0-9a-zA-Z]" ":;.,!?")
 (nocolon "--9$+<>@-Z_[:alpha:]~" "<@" "@>;.,!?")
 (machine "-[:alnum:]." "" ".")
 (math-mode ",-:$+<>@-Z_[:lower:]~`" "<" "@>;.,!?`:")
 (latex-mode "--:\\$+<>@-Z_[:alpha:]~*?" "<@" "@>;.,!:")
 (tex-mode "--:\\$+<>@-Z_[:alpha:]~*?" "<@" "@>;.,!:"))


  This variable may be risky if used as a file-local variable.

Documentation:
Alist of (MODE CHARS BEG END), where MODE is a symbol,
possibly a major-mode name, or one of the symbols
‘file’, ‘url’, ‘machine’, and ‘nocolon’.
Function ‘ffap-string-at-point’ uses the data fields as follows:
1. find a maximal string of CHARS around point,
2. strip BEG chars before point from the beginning,
3. strip END chars after point from the end.
The arguments CHARS, BEG and END are handled as described in
‘skip-chars-forward’.