const regex = /(?:(if\s+)|\|\|\s*)(?<attrName>[\p{L}\p{N}_-]+)(?==")/gu;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('(?:(if\\s+)|\\|\\|\\s*)(?<attrName>[\\p{L}\\p{N}_-]+)(?==")', 'gu')
const str = `<if VERSION="A1" || VERSION="A3"||VERSION="a2_%5C%22A3">
<assign CTA="blue"></assign>
<assign CTA2="green"></assign>
<assign TEXT1="Hello%3Cbr/%3EWorld"></assign>
<elseif attribute-name="A2">
<assign CTA="red"></assign>
<assign CTA2="yellow"></assign>
<assign CTA3="brown"></assign>
<assign TEXT1="Click%20%3Ca%20href='https://example.com'%20style='text-decoration:none;color:#000000;'%3Ehere%3C/a%3E"></assign>
</if>
// condition 2
<if FOO_BAR="A4" || baz-biz="A5">
<assign CTA="purple"></assign>
<assign CTA2="orange"></assign>
<assign TEXT1="Hi%20%3Cspan%20style='font-%20weight:bold;'%3EJohn%3C/span%3E" TEXT2="Hello%3Cbr/%3EWorld"></assign>
</if>
// condition 3
<if LANG="en_US">
<assign TITLE="English"></assign>
</if>`;
const subst = `$1 $<attrName>`;
// 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