elrata_

NOTE: I’m looking for a new maintainer. Please reach out to 
rodrigo@sdfg.com.ar if you are interested.
NOTE: I don’t have wordpress 5 installations to try this and plan to stop 
updating it. If this is a big issue for you, please let me know so I can 
consider to continue doing it. Also, let me know if you are willing to help with testing :).
This is a small modification of the wp-recaptcha plugin to make it work with 
BuddyPress. Currently uses recaptcha v2.
You only need this plugin if you use Buddypress with wordpress and want a 
recaptcha at the registration page.
It is always rebased on top of wp-recaptcha. See 
https://github.com/rata/wp-recaptcha-bp for more info
What is reCAPTCHA?
reCAPTCHA is a free CAPTCHA service that protects your site against spam, malicious registrations and other forms of attacks where computers try to disguise themselves as a human; a CAPTCHA is a Completely Automated Public Turing test to tell Computers and Human Apart. reCAPTCHA comes in the form of a widget that you can easily add to your blog, forum, registration form, etc.
In addition to protecting your site, reCAPTCHA also helps us digitize old books and newspapers, transcribe street numbers and solve hard AI problems. <a href="http://www.google.com/recaptcha" rel="nofollow ugc">Learn how reCAPTCHA works</a> and join our <a href="https://groups.google.com/forum/#!forum/recaptcha" rel="nofollow ugc">forum</a>.
<h3>Requirements</h3>
<ul>
<li>You need the reCAPTCHA keys <a href="https://www.google.com/recaptcha/admin#whyrecaptcha" rel="nofollow ugc">here</a>.</li>
<li>Your theme must have a <code>do_action('comment_form', $post->ID);</code> call right before the end of your form (Right before the closing form tag). Most themes do.</li>
</ul>

WP-reCAPTCHA-bp

Integrates reCAPTCHA anti-spam methods with WordPress sites using BuddyPress plugin, including comment and registration spam protection.

jmviade

reCAPTCHA in WP comments form plugin is an ANTISPAM tool that adds the visible Google reCAPTCHA field inside the comments form of your WP theme when the user is not logged in preventing fraudulent or deceptive comments.
The plugin also introduces a second verification process that detects the unauthorized direct accesses by spam robots to the WP comments system and allows you to decide what do you want to do with those comments.
Finally, the plugin has got an optional forced javascript output mode that lets you to add a reCAPTCHA field also in old WP themes that didn’t use the new WP form comments functions but they make a direct output of its own comments form.
<h4>FEATURES LIST</h4>
Basic Features
<ul>
<li>All variants of Google reCAPTCHA field are available</li>
<li>Two simple steps Installation Wizard</li>
<li>Automatic default configuration settings for all plugin components</li>
<li>Automatic default configuration for reCAPTCHA field</li>
<li>Configuration settings for Plugin </li>
<li>Configuration settings for ANTISPAM operation</li>
<li>Four modes of operation in case of spam robots threats (SPAM, TRASH, DELETE or DIE)</li>
<li>Visual configuration settings for Google reCAPTCHA: theme, size, type, align, language</li>
<li>Dynamic comments form sample for viewing configuration settings changes</li>
<li>Visual Help</li>
<li>RTL Language support</li>
<li>Admin Color scheme adapted</li>
</ul>
Middle features
<ul>
<li>Forced language option for reCAPTCHA field</li>
<li>Plugin blocks the submit button while reCAPTCHA field is not verified</li>
<li>Plugin changes HTML structure of the comments form to prevent malicious automatic sendings while reCAPTCHA field is not verified</li>
<li>Plugin also blocks other elements with <code>[type=submit]</code> inside form in case of a theme customized comments form</li>
<li>Plugin lets you to write your own additional CSS for the reCAPTCHA field</li>
<li>New restore default value buttons in plugin configuration section for helping you in case of changing WP theme, accidental errors, test environtments, etc.</li>
</ul>
Advanced features
<ul>
<li>reCAPTCHA verification process via AJAX before submitting the form</li>
<li>Second security checking process for preventing any security breach before saving the comment</li>
<li>Supporting four different WP comments form HTML structure types</li>
<li>Advanced plugin options based on HTML queries for inserting the reCAPTCHA plugin in all kinds of WP themes</li>
<li>Optional Forced javascript output that allows you to use the plugin with old WP themes that didn’t use function <code>comment_form()</code></li>
<li>Advanced ID’s tags settings for using this plugin with WP Themes that creates its own comments form HTML struct</li>
<li>reCAPTCHA javascript initialization that prevents reCAPTCHA conflicts in case of that other plugins use reCAPTCHA.</li>
<li>New mínimum CSS styles for recaptcha alignment</li>
</ul>
<h4>PLUGIN PAGE</h4>
To learn more about the plugin, visit the <a href="http://www.joanmiquelviade.com/plugin/google-recaptcha-in-wp-comments-form/" title="Author's plugin page" rel="nofollow ugc">Plugin page</a>.

reCAPTCHA in WP comments form

reCAPTCHA in WP comments form is an ANTISPAM tool that adds a Google reCAPTCHA to the comments form and protects your site from the spam robots threat …

WP-reCAPTCHA-bp vs reCAPTCHA in WP comments form