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In order to insert after the <body> tag, we must first find it. We can use the regular expression pattern <body.*?> for that.

In this task we don’t need to modify the <body> tag. We only need to add the text after it.

Here’s how we can do it:

let str = '...<body style="...">...';
str = str.replace(/<body.*?>/, '$&<h1>Hello</h1>');

alert(str); // ...<body style="..."><h1>Hello</h1>...

In the replacement string $& means the match itself, that is, the part of the source text that corresponds to <body.*?>. It gets replaced by itself plus <h1>Hello</h1>.

An alternative is to use lookbehind:

let str = '...<body style="...">...';
str = str.replace(/(?<=<body.*?>)/, `<h1>Hello</h1>`);

alert(str); // ...<body style="..."><h1>Hello</h1>...

As you can see, there’s only lookbehind part in this regexp.

It works like this:

  • At every position in the text.
  • Check if it’s preceeded by <body.*?>.
  • If it’s so then we have the match.

The tag <body.*?> won’t be returned. The result of this regexp is literally an empty string, but it matches only at positions preceeded by <body.*?>.

So it replaces the “empty line”, preceeded by <body.*?>, with <h1>Hello</h1>. That’s the insertion after <body>.

P.S. Regexp flags, such as s and i can also be useful: /<body.*?>/si. The s flag makes the dot . match a newline character, and i flag makes <body> also match <BODY> case-insensitively.