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

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.