There are two ways to approach this problem:
- Understand the regex (way harder than necessary) and fix it to your specification (may be buggy)
- Who cares how the regex does its thing (it seems to do what you need) and modify your input to conform to what you think the regex does
The second approach means that you just check if the input string ends with \. If it doesn't then just add it on, then let the regex does it magic.
I normally wouldn't recommend this ignorant alternative, but this may be an exception.
Blackboxing
Here's how I'm "solving" this problem:
- There's a magic box, who knows how it works but it does 99% of the time
- We want it to work 100% of the time
- It's simpler to fix the 1% so it works with the magic box rather than fixing the magic box itself (because this would require understanding of how the magic box works)
- Then just fix the 1% manually and leave the magic box alone
Deciphering the black magic
That said, we can certainly try to take a look at the regex. Here's the same pattern but reformatted in free-spacing/comments mode, i.e. (?x) in e.g. Java.
^
( [A-Za-z]:
| \\{2} ( [-\w]+
| (
(25[0-5]
|2[0-4][0-9]
|[01]?[0-9][0-9]?
)\.
){3}
(25[0-5]
|2[0-4][0-9]
|[01]?[0-9][0-9]?
)
)
\\ (
( [^"*/:?|<>\\,;[\]+=.\x00-\x20]
| \.[.\x20]* [^"*/:?|<>\\,;[\]+=.\x00-\x20]
)
( [^"*/:?|<>\\,;[\]+=\x00-\x1F]*
[^"*/:?|<>\\,;[\]+=\x00-\x20]
)?
)
)
\\ (
[^"*/:?|<>\\.\x00-\x20]
(
[^"*/:?|<>\\\x00-\x1F]*
[^"*/:?|<>\\.\x00-\x20]
)?
\\
)*
$
The main skeleton of the pattern is as follows:
^
(head)
\\ (
bodypart
\\
)*
$
Based from this higher-level view, it looks like an optional trailing \ can be supported by adding ? on the two \\ following the (head) part:
^
(head)
\\?(
bodypart
\\?
)*
$
References
Note on catastrophic backtracking
You should generally be very wary of nesting repetition modifiers (a ? inside a * in this case), but for this specific pattern it's "okay", because the bodypart doesn't match \.
References