Insert After Head

We have a string with an HTML Document.

Write a regular expression that inserts <h1>Hello</h1> immediately after <body> tag. The tag may have attributes.

For instance:

 
             let regexp = /your regular expression/;

let str = `
<html>
  <body style="height: 200px">
  ...
  </body>
</html>
`;

str = str.replace(regexp, `<h1>Hello</h1>`); 
            

After that the value of str should be:

 
             <html>
  <body style="height: 200px"><h1>Hello</h1>
  ...
  </body>
</html> 
            

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.