Want to use Regex to dynamically encode ampersand in urls

Hi!
I've been scouring the web for an answer and I think my limitation is that I'm not that familiar with Regular Expressions and how they work. Especially, how to add the code to my loop or template file.

I found this code:

text = Regex.Replace(text, @"
    # Match & that is not part of an HTML entity.
    &                  # Match literal &.
    (?!                # But only if it is NOT...
      \w+;             # an alphanumeric entity,
    | \#[0-9]+;        # or a decimal entity,
    | \#x[0-9A-F]+;    # or a hexadecimal entity.
    )                  # End negative lookahead.", 
    "&",
    RegexOptions.IgnoreCase | RegexOptions.IgnorePatternWhitespace);

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But I don't know how to add it to my file. I have a custom field for a Indiebound link. Most of the links contain the ampersand, so code isn't validationg. Here is the section of code:

if ( ! is_active_sidebar( 'sidebar-books' ) ) {
	return;
}
?>

<div id="buy-it" class="widget-area" role="complementary">
	<?php if (is_single() && is_post_type('book')) : ?>
	<aside class="beige buy">
		<h2>Buy It!</h2>
		<ul>
			<?php if(get_field('indie_bookstores_link') !=false) { ?> 
			<li><a href="<?php the_field('indie_bookstores_link'); ?>" target="_blank"><img src="<?php bloginfo('url'); ?>/wp-content/uploads/2015/08/indiebound.png"></a></li><?php } ?>
			<?php if(get_field('amazon_link') !=false) { ?>
			<li><a href="<?php the_field('amazon_link'); ?>" target="_blank"><img src="<?php bloginfo('url'); ?>/wp-content/uploads/2015/08/amazon.png"></a></li><?php } ?>
			<?php if(get_field('barnes_n_noble_link') !=false) { ?>
			<li><a href="<?php the_field('barnes_n_noble_link'); ?>" target="_blank"><img src="<?php bloginfo('url'); ?>/wp-content/uploads/2015/08/barnes-noble.png"></a></li><?php } ?>
		</ul>
	</aside>
	<?php endif; ?>
</div><!-- #buy-it -->

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