Regular expression with optional part and negative lookahead

问题

I need check if a string contains the pattern: starts with "A" followed by zero or more spaces and then anything but not "B".

So, the following must match: "A". "AX", "A X", "A ", "A XB"

The following strings must not match: "AB", "A B"

My naive attempt was A\s*(?!B), but it matches the undesirable "A B".

回答1:

If you just need to get true or false, you may put the \s* into the lookahead:

Regex.IsMatch(s, @"A(?!\s*B)")

It finds A that has no 0+ whitespaces followed with B after it.

See the regex demo.

In your pattern, A\s*(?!B), the negative lookahead can be executed after any 0+ whitespaces, and once a whitespace not followed with B is found, a valid match is returned (that happens due to backtracking that is possible thanks to \s* quantified pattern).

If you need to actually match the A and the whitespace after it, but if these whitespaces are not followed with B, use the pattern from my comment.

(?>A\s*)(?!B)

This pattern matches:

• (?>A\s*) - an atomic group, matches A, then 0+ whitespaces with no backtracking into the group pattern allowed
• (?!B) - no B after the spaces, or the whole match is failed.

回答2:

Update: Based on the comment below, use this pattern A\s*B|(A) and check against group #1

---

Use this pattern A\s*+(?!B)\w* Demo

#    A\s*+(?!B)\w*
A           # "A"
\s          # <whitespace character>
*+          # (zero or more)(possessive)
(?!         # Negative Look-Ahead
  B         # "B"
)           # End of Negative Look-Ahead
\w          # <ASCII letter, digit or underscore>
*           # (zero or more)(greedy)

or based on your attempt, use this A\s*+(?!B)

回答3:

The attempt doesn't work because it matches the "A" before the " B", I would suggest attempting to take the negative approach, and writing A\s*B and use all non-matching lines. For example with grep grep -v "A\\s*B".

If the A is needed and there are non-A lines you can search twice, grep A | grep -v "A\\s*B".

来源：https://stackoverflow.com/questions/45463437/regular-expression-with-optional-part-and-negative-lookahead