Why does my regex in JavaScript return undefined?

Question

I'm trying to find all the matches for 'test' in my string:

 const search = "test"; const regexString = "(?:[^ ]+ ){0,3}" + "test" + "(?: [^ ]+){0,3}"; const re = new RegExp(regexString, "gi"); const matches = []; const fullText = "my test string with a lot of tests that should match the test regex"; let match = re.exec(fullText); while (match != undefined) { matches.push(match[1]); match = re.exec(fullText); } console.log(matches); 

I'm getting the following:

[ undefined, undefined, undefined ]

Why isn't my search working?

Answer 1

Your code expects the result of the match to include stuff captured in capturing groups in the regular expression. However, your regular expression contains only non-capturing groups. The (?: ) grouping explicitly does not capture the matched substring.

You want plain ( ) groupings.

Answer 2

You should enclose your non-capturing groups (?:...) in a capturing group (...) since you are invoking a capturing group ( match[1] ). :

"((?:\\S+ ){0,3})" + search + "((?: \\S+){0,3})"

Trying to return an array containing the 3 words preceding and proceeding 'test'

Then you need to push both captured groups not one:

matches.push([match[1], search, match[2]]);
// `match[1]` refers to first capturing group
// `match[2]` refers to second CG
// `search` contains search word

JS code:

 const search = "test"; const regexString = "((?:\\\\S+ ){0,3})" + search + "((?: \\\\S+){0,3})"; const re = new RegExp(regexString, "gi"); const matches = []; const fullText = "my test string with a lot of tests that should match the test regex"; while ((match = re.exec(fullText)) != null) { matches.push([match[1], search, match[2]]); } console.log(matches);