\b（？！（？：。\B）*（。）（?: \B。）* \ 1）[abc] + \ b 
 

您可以使用此与任何设置和大小的模式，只需用所需的设置替换 [abc] ...
$ b $ hr

示例：

（以上输出来自 myregextester ）

How can I match letters a,b,c once in any combination and varying length like this:

The expression should match these cases:

abc
bc
a
b
bca

but should not match these ones:

abz
aab
cc
x

Use regex pattern

\b(?!(?:.\B)*(.)(?:\B.)*\1)[abc]+\b

You can use this pattern with any set and size, just replace [abc] with desired set...

Example:

(above output is from myregextester)