javascript - Regex for if a character exists after the last instance of a character

Question

I'm trying to put together a regex that will indicate whether a character exists after the last instance of a character in a string. For example, the regex would return a match if a period '.' appeared after the last instance of a '/'

So far I am able to find the string combination for the final part of the string after the / using:

   [^/]+$

But I am not sure how to only return a match if a period '.' is included in that final string portion. I greatly appreciate any help.

I realize it would be possible to do this by splitting the string, but I was hoping a pure regex way existed.

question from:https://stackoverflow.com/questions/65891518/regex-for-if-a-character-exists-after-the-last-instance-of-a-character

Answer 1

You can use

/[^/.]*.[^/]*$/       // The whole match is the required result
/.*/([^/.]*.[^/]*)$/ // and extract Group 1 contents

See the regex demo #1 and regex demo #2.

The second one is more efficient in practice, as the part after last / is usually closer to the end of a longer string, and the match is usually obtained faster.

Details

• [^/.]* - zero or more chars other than . and /
• . - a dot
• [^/]* - zero or more chars other than /
• $ - end of string.

JavaScript demo:

const texts = [ 'a/b/c', 'a/b/c.a' ];
texts.forEach( text => {
    console.log( text, '(/[^/.]*.[^/]*$/) =>', (text.match(/[^/.]*.[^/]*$/)?.[0] || "No match") );
    console.log( text, '(/(.*/([^/.]*.[^/]*)$/) =>', (text.match(/.*/([^/.]*.[^/]*)$/)?.[1] || "No match") );
  }
)