任何方式匹配模式EITHER先于OR后跟某个字符？

Question

For instance, I'd like to match all of the following strings:

'abc' 'cba' 'bca' 'cab' 'racb' 'rcab' 'bacr'

but NOT any of the following:

'rabcr' 'rbacr' 'rbcar'

Is this possible with regex?

Answer 1

The easiest way would be to use alternation :

/^(?:[abc]{3}r?|r[abc]{3})$/

Explanation:

^         # Start of string
(?:       # Non-capturing group:
 [abc]{3} # Either match abc,cba,bac etc.
 r?       # optionally followed by r
|         # or
 r        # match r
 [abc]{3} # followed by abc,cba,bac etc.
)         # End of group
$         # End of string

Some regex engines support conditionals , but JavaScript is not among them. But in .NET, you could do

^(r)?[abc]{3}(?(1)|r?)$

without the need to write your character class twice in the same regex.

Explanation:

^        # Start of string
(r)?     # Match r in group 1, but make the group optional
[abc]{3} # Match abc,cab etc.
(?(1)    # If group 1 participated in the match,
         # then match nothing,
|        # else
 r?      # match r (or nothing)
)        # End of conditional
$        # End of string

Another solution in JavaScript would be to use a negative lookahead assertion :

/^(?:r(?!.*r$))?[abc]{3}r?$/

Explanation:

^         # Start of string
(?:       # Non-capturing group:
 r        # Match r
 (?!.*r$) # only if the string doesn't end in r
)?        # Make the group optional
[abc]{3}  # Match abc etc.
r?        # Match r (optionally)
$         # End of string