Filter out substrings that include keywords in a text in Javascript with Regex

Question

I have a solve a complex problem: I want to filter out substrings from a possible long text. There are certain keywords that indicate a substring. Only if a keyword is preceeded by at least one character, which is not a white space or a different keyword, should match. Then also every character they keyword is preceeded by should be included in the match. I want to use a regex expression in JavaScript for this.

My keywords are: ":yellow:", ":black:", ":green:", ":blue:" ":red:"

For example I have a Text like this: " :green: aba :red: gd efg:blue: :yellow: sdg:red: sea gea e :black: "

Now I want to use match() on this string with a re that gives me these matches: " aba :red:", "gd efg:blue":, "sdg:red:", sea gea e :black:

:green: at the start should not be matched, because it is not preceded by a character. :yellow: should also not be matched, because it is preceded by a different keyword (in this case :blue:)

I have tried to use negative lookahead expressions (like (?!)) to prevent matching when keywords preceed other keywords. But it didn't quite give me the results I am looking for.

    /((?!(:yellow:|:black:|:green:|:blue:|:red:))\S+\s*)+(:yellow:|:black:|:green:|:blue:|:red:)/g
    
    let ar1 = text1.match(re1);
    
    console.log(ar1);

this is my output: [ 'green: aba :red:', 'gd efg:blue: :yellow:', 'sdg:red: sea gea e :black:' ]

but i want this:

[ ' aba :red:', 'gd efg:blue: ', 'sdg:red:', 'sea gea e :black:' ]

The fourth bird · Accepted Answer · 2019-07-15 21:39:09Z

You could shorten your alternation by placing the : outside of it and instead of matching \S+ you could match not a whitespace char or a : using a negated character class.

To match the multiple "words" you could repeat matching a space and use the negated character class again.

(?!:(?:yellow|black|green|blue|red):)[^\s:]+(?: [^\s:]+)*\s*:(?:yellow|black|green|blue|red):

Explanation

(?! Negative lookahead, assert what is directly on the right is not
- :(?:yellow|black|green|blue|red): Match any of the listed between :
) Close negative lookahead
[^\s:]+ Match 1+ times not a whitespace char or :
(?: [^\s:]+)* Repeat 0+ times matching a space, then 1+ times not a whitespace char or :
\s* Match 0+ whitespace chars
:(?:yellow|black|green|blue|red): Match any of the listed between :

Regex demo

const regex = /(?!:(?:yellow|black|green|blue|red):|\s)[^\s:]+(?: [^\s:]+)*\s*:(?:yellow|black|green|blue|red):/g;
const str = ` :green: aba :red: gd efg:blue: :yellow: sdg:red: sea gea e :black: `;
console.log(str.match(regex));

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