CODEWITHSUNDEEP
javascriptregex
Kpower
Kpower

Reputation: 1257

Javascript:Replacing letters within certain phrases only

Over here I was asked to form a new question with one of my comments so here I am. I was wondering if it was possible to replace a phrase within certain words only. Eg: Replacing the BAB in CBABAC but not the BAB in DABABCC Thanks!

Upvotes: 1

Views: 76

Answers (2)

Mitya
Mitya

Reputation: 34556

You don't say what the basis is for the logic of the replacement, so this is a generalised answer.

As some have mentioned, you could use a lookahead, but one of the major annoyances of JavaScript is that it doesn't natively support lookbehinds, so you have only half a solution there.

A common workaround for the lack of look-behind is to match (rather than anchor to) what comes before the bit you're interested in, then reinsert it in the callback.

Let's say I want to replace all instances of foo with bar, where it is preceded and proceded by a number.

var str = 'foo 1foo1 foo2';
console.log(str.replace(/(\d)foo(?=\d)/g, function($0, $1) {
    return $1+'bar';
})); //foo 1bar1 foo1

So I use a lookahead for the easy part, and a callback to compensate for the lack of lookbehind.

There are implementations of lookbehind in JS, including one I wrote, where a positive or negative lookbehind is passed as an extra parameter. Using that, this would get the same result as the above:

console.log(str.replace2(/foo(?=\d)/g, 'bar', '(?<=\\d)'));

Upvotes: 0

Cylian
Cylian

Reputation: 11182

Use lookahead:

BAB(?=AC)

Explanation

"BAB" +      // Match the characters “BAB” literally
"(?=" +      // Assert that the regex below can be matched, starting at this position (positive lookahead)
   "AC" +       // Match the characters “AC” literally
")"

or 

BAB(?!CC)

Explanation

"BAB" +      // Match the characters “BAB” literally
"(?!" +      // Assert that it is impossible to match the regex below starting at this position (negative lookahead)
   "CC" +       // Match the characters “CC” literally
")"

Upvotes: 2

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