const regex = /<assign.*?>/g;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('<assign.*?>', 'g')
const str = `<if VERSION="A1" || VERSION="A3" || VERSION="a2_%5C%22A3">
<assign CTA="blue">
<assign CTA2="green">
<assign TEXT1="Hello%3Cbr/%3EWorld">
<elseif attribute-name="A2">
<assign CTA="red">
<assign CTA2="yellow">
<assign CTA3="brown">
<assign TEXT1="Click%20%3Ca%20href='https://example.com'%20style='text-decoration:none;color:#000000;'%3Ehere%3C/a%3E">
</if>
// condition 2
<if FOO_BAR="A4" || baz-biz="A5">
<assign CTA="purple">
<assign CTA2="orange">
<assign TEXT1="Hi%20%3Cspan%20style='font-%20weight:bold;'%3EJohn%3C/span%3E" TEXT2="Hello%3Cbr/%3EWorld">
</if>
// condition 3
<if LANG="en_US">
<assign TITLE="English">
</if>`;
const subst = `$&</assign>`;
// The substituted value will be contained in the result variable
const result = str.replace(regex, subst);
console.log('Substitution result: ', result);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for JavaScript, please visit: https://developer.mozilla.org/en/docs/Web/JavaScript/Guide/Regular_Expressions