Redirects and regular expressions (RegEx)"

In the world of SEO, one of the most common actions we apply to a project is redirects. From removing products from an online store to website changes, this mechanism is essential to prevent losing the accumulated ranking of one or more URLs.

In large websites, the process of preparing these redirects can take forever if many URLs have been modified and it hasn’t been carefully planned. Imagine, for example, redesigning a web directory with thousands of references where all the URLs of listings and pages have been changed. It would be practically impossible to create all the redirects manually.

In these types of scenarios, regular expressions come into play: through them, we can greatly speed up the task of generating redirects in large projects. Let's define what they are and look at real examples of using regular expressions for redirects.

What are redirects?

Strictly speaking, redirects in the web environment are a group of HTTP response codes (specifically, the 300s). They are used to inform our browser that the URL we are requesting is no longer available, while providing us with a new URL to access. This process is usually transparent to the end user, as the browser handles (re)directing us to the new URL.

The 300 response codes are as follows:

• 300 Multiple Choices
• 301 Moved Permanently
• 302 Found
• 303 See Other
• 304 Not Modified
• 305 Use Proxy Deprecated
• 306 unused
• 307 Temporary Redirect
• 308 Permanent Redirect

In practice, when we talk about a redirect in the SEO field, we usually refer to 301 redirects. Why? They are the ones we typically need to use to transfer SEO value from one page to another, and they are recommended by Google for permanent redirects.

301 redirects consist of two parts:

• Response header, where the "301 Moved Permanently" redirect code is indicated.
• The location to which we want to redirect the user, via the "Location" field.

What are regular expressions?

Regular expressions, or RegEx, are patterns of different characters (letters, numbers, symbols) that we use to analyze a text string. For example, in a form where one of the fields is the DNI, we can use a regular expression to determine if it has the correct format. Or when a user enters a date, we can extract the year from it.

The combinations of patterns in regular expressions are infinite, and it’s hard to get lost when trying to set one up that’s useful for us. In practice, the simpler, the better, as long as the project we’re working on allows it. Some common patterns we can use in RegEx are:

• .* (equivalent to any character, with 0 or more occurrences)
• a|b (to identify that either a or b can appear)
• ^ (to indicate the beginning of the text string)
• $ (to indicate the end of the text string)
• Literal text: in some cases, we may want to indicate a pattern identical to the text we entered, like a specific word.

How to apply regular expressions in redirects?

We’ve already seen the definition of both regular expressions and redirects. But in practice, how can we use RegEx for our redirects? Depending on the type of website we have, we should choose one solution or another:

• htaccess file: this file manages the routes of a website, and we can apply redirects with regular expressions within it.
• Redirect plugin or module: if we use a web CMS like WordPress or Prestashop.
• Native code: some programming languages allow applying redirects with RegEx directly in the code.

For this article, we will focus on redirects applied with the htaccess file. However, the logic behind them would be the same if we used another method.

Real-life examples of regular expressions in redirects

Full domain redirect

In the case where our website has only changed domains, we can point all the URLs from the old domain to the new one with this rule in our htaccess:

RewriteRule ^(.*)$ https://www.mydomain.com/$1 [R=301,L]

• .* indicates that we need to capture any character.
• The parentheses () are used to store the identified content (we’ll see why shortly)
• The characters ^ and $ indicate the beginning and end of the text string

• With https://www.mydomain.com/ we define the new domain
• $1 is the content that was within the parentheses in the first part of the redirect

Subdomain redirect

When we have multiple subdomains (e.g., www), we may want to unify them all under the same subdomain or our main domain. Here we need to add a condition to apply the redirect only to the subdomains we are interested in:

RewriteCond %{HTTP_HOST} ^www\.mydomain\.com$ [NC]

RewriteRule ^(.*)$ https://mydomain.com/$1 [R=301,L]

• The first line is the condition, which restricts the redirect to only apply when the subdomain is www.mydomain.com
• The second part is the redirect itself, just as we saw in the previous example.

Redirect with source pattern

Let’s say we want to redirect all URLs on our website that contain a specific word:

RewriteRule ^.*word.*$ /destination [R=301,L]

• In the first part of the redirect, we use the regular expression ^.*word.*$ to indicate that we want to select URLs that start with any character (^.*), followed by the literal word, and end with any character (.*$).
• In the second part, we literally indicate the URL we want to redirect to.

Redirect with folder change

Now, let’s imagine that while working on an online shoe store, the URL for the Sports Shoes category has changed, from /shoes/sport/ to /shoes/sports/. And this affects all the product URLs in that category, like /shoes/sport/adidas-low-forum/. To manage these redirects with regular expressions, we can use the following code:

RewriteRule ^/shoes/sport/(.*)$ /shoes/sports/$1 [R=301,L]

• In the first part of the redirect ^/shoes/sport/(.*)$, we indicate that we want to affect all URLs that start with ^/shoes/sport/, and then capture the final part of the URL with (.*)$
• In the second part, we use /shoes/sports/ to add the new category route, and $1 to retrieve the final product path.

These are just a few real examples of redirects using regular expressions. As mentioned earlier, the flexibility of regular expressions is practically infinite. But if you’re unsure and would prefer a professional team to help with your project, our SEO agency will be happy to assist you.