How to get a surrounding text for a found string in JavaScript?

Question

This is a noob question

Suppose I search a string S for a pattern P . Now I would like to display a substring of the string, which surrounds P . The substring should be only one line (ie N characters) and contain whole words. How would you code it in JavaScript ?

For example:
Let S = "Hello world, welcome to the universe", P = "welcome", and N = 15. The naive solution gives "ld, welcome to " (adding 4 chars before and after P ). I would like to "round" it up to "world, welcome to".

Can regular expressions help me here?

Answer 1

Here is the regular expression you wanted:

/\s?([^\s]+\swelcome\s[^\s]+)\s?/i    //very simple, no a strange bunch of [] and {}

Explanation:

What you are trying to match is actually

" world, welcome to "

without the spaces before and after, therefore:

\s?       //the first space (if found)
(         //define the string position you want
[^\s]+    //any text (first word before "welcome", no space)
\s        //a space
welcome   //you word
\s        //a space
[^\s]+    //the next world (no space inside)
)         //that's it, I don't want the last space
\s?       //the space at the end (if found)

Applying:

function find_it(p){
    var s = "Hello world, welcome to the universe",
        reg = new RegExp("\\s?([^\\s]+\\s" + p + "\\s[^\\s]+)\\s?", "i");

    return s.match(reg) && s.match(reg)[1];
}

find_it("welcome");   //"world, welcome to"

find_it("world,");    //"Hello world, welcome"

find_it("universe");  //null (because there is no word after "universe")

Answer 2

I think, this is, what your are looking for.

$a = ($n - length of $p)/2
/[a-zA-Z0-9]{$a}$p[a-zA-Z0-9]{$a}/

I used dollars to show where the variables are. You didn't provide enough code to write a concrete example.