How to only match a single instance of a character?

Question

Not quite sure how to go about this, but basically what I want to do is match a character, say a for example. In this case all of the following would not contain ma

Answer 1

Basically what I want to do is match any single a that has any other non a character around it (except for the start and end of the string).

^[^\sa]*\Ka(?=[^\sa]*$)

DEMO

\K discards the previously matched characters and lookahead assertes whether a match is possibel or not. So the above matches only the letter a which satifies the conditions.

OR

a{2,}(*SKIP)(*F)|a

DEMO

Answer 2

You need two things:

• a negated character class: [^a] (all except "a")
• anchors (^ and $) to ensure that the limits of the string are reached (in other words, that the pattern matches the whole string and not only a substring):

Result:

^[^a]*a[^a]*$

Once you know there is only one "a", you can use the way you want to extract/replace/remove it depending of the language you use.

Answer 3

You may use a combination of a lookbehind and a lookahead:

(?<!a)a(?!a)

See the regex demo and the regex graph:

Details

• (?<!a) - a negative lookbehind that fails the match if, immediately to the left of the current location, there is a a char
• a - an a char
• (?!a) - a negative lookahead that fails the match if, immediately to the right of the current location, there is a a char.