Regular expressions (also known as REGEX or REGEXP) help you find URLs or text that match a particular pattern.

Let’s move from the simplest examples to more advanced ones.

The following configuration includes all URLs that contain the word “blog” and exclude URLs containing the word “product.”

This instructs our bot to crawl:
​https://ahrefs.com/blog
​https://ahrefs.com/blog/seo-techniques/
​https://ahrefs.com/academy/blogging-for-busines

And to ignore:
​https://ahrefs.com/blog/category/product-blog/
​https://ahrefs.com/blog/ecommerce-out-of-stock-products/

That was easy, right?

But what if you want to include the URLs from /blog/ subfolder specifically but not https://ahrefs.com/academy/blogging-for-business?

You can use a bit more advanced pattern:

You probably wonder what those symbols before and after “blog” are. 

In the regex, you have to “escape” some symbols so that they are not recognized as special characters. To do that, use the backslash \ before the character.

A simple dot . in the regex, for example, stands for any character. But \. works as a full stop symbol. That’s why I escaped the slash character in the example above like this: \/

Please note that you can apply multiple patterns to include or exclude URLs in the crawl settings in Site Audit. If no matches are found for one of the rules, the rule will be skipped.

The rules above will instruct our crawler to crawl the URLs that contain the words:

“blog” or “product”

AND do not contain the words:

“blogging” or “productive.”

For multiple URL rewrite rules, each new rule is executed sequentially to the result of previous rewrite.

^ - this symbol indicates the beginning of the URL

$ - this symbol indicates the end of the URL

. - the decimal point matches any single character

* - matches the preceding expression 0 or more times

+ - matches 1 or more of the previous

? - Matches 0 or 1 of the previous

| - is equivalent to OR.

[__] - is similar to |, but can be used to define ranges

(__) - parentheses group the regex between them

\d - matches one digit

\D - matches one non-digit

\w - matches a word character

\W - matches a non-word character

Different tools and platforms may be using different regex libraries. Our Site Audit is using RE2. You can find its full syntax here.

^https:.*\/wp-content\/

^ indicates the beginning of the URL. This rule will match all URLs that start with “https:” followed by 0 to any number of characters .* before “/wp-content/”.

URLs matching the pattern:
​https://ahrefs.com/blog/wp-content/uploads/2019/03/fb-ranking-1-image-1.png

URLs not matching the pattern:
​http://ahrefs.com/blog/wp-content/uploads/2019/03/fb-ranking-1-image-1.png

\/blog(\/.*)?$

This rule will match all URLs that end $ with “/blog”, optionally followed by the slash and 0 to any number of characters (\/.*)?. The question mark in this pattern matches the expression in the brackets between zero and one time, making it optional.

URLs matching the pattern:
​https://ahrefs.com/blog
​https://ahrefs.com/blog/301-redirects/

URLs not matching the pattern:
​https://ahrefs.com/blogging
​https://ahrefs.com/academy/blogging-for-business

@|% or [@%] 

| and [__] work as OR

URLs matching the pattern:
​https://ahrefs.com/@timsoulo
​https://ahrefs.com/%D1%81%D0%B5%D0%BE

URLs not matching the pattern:
​https://ahrefs.com/blog/nofollow-links

\?add-to-cart=

Keep in mind that ? is a special symbol in the regex. To use it as a simple question mark, don’t forget to escape it like this: \?

URLs matching the pattern:
​https://yourdomain.com/?add-to-cart=25

URLs not matching the pattern:
​https://yourdomain.com/smartphones

[0-9]{4} or \d{4}

[0-9]{4} will match all URLs containing four {4} digits [0-9] in a row

\d{4} does the same as \d stands for one digit

URLs matching the pattern:
​https://yourdomain.com/best-smartphones-2019

URLs not matching the pattern:
​https://yourdomain.com/smartphones

^https?:\/\/help.ahrefs.com

This rule will match all URLs that begin with "http://help.ahrefs.com" or "https://help.ahrefs.com".

The question mark here s? indicates that “s” is optional, so both http and https will match this rule.

URLs matching the pattern:
​https://help.ahrefs.com
​http://help.ahrefs.com/
​http://help.ahrefs.com/site-audit

URLs not matching the pattern:
​https://ahrefs.com/site-audit
​ftp://help.ahrefs.com

\.(jpg|gif|bmp|png|css|pdf)$

This rule will match all URLs that end $ with .jpg OR .gif OR .bmp OR .png OR .css OR .pdf.

You can reduce this expression to target a smaller number of extensions. For example, for only URLs that end $ with .jpg OR .png only:

​ .*\.(jpg|png)$

Parentheses (__) group the regex between them and | stands for OR

URLs matching the pattern:
​https://ahrefs.com/blog/wp-content/uploads/2019/03/fb-ranking-1-image-1.png
​http://ahrefs.com/blog/wp-content/uploads/2019/03/fb-ranking-1-image-1.png 

URLs not matching the pattern:
​https://ahrefs.com/site-audit

You can use URL rewrite rules in Site Audit settings to replace some parts of the URLs with another value.

E.g. When your staging website is based on a subdomain, e.g. staging.ahrefs.com

This rule will replace "ahrefs.com" in every URL with "staging.ahrefs.com"

With capturing groups, you can replace several parts of the URL with one rule.

Pattern to match:

www\.ahrefs\.com([^\?|#]*)?([#]?[^\?]*)\??(.*)

Replace with:

www.ahrefs.com\1?parameter1=5273&\2?parameter2=7465&\3

([^\?|#]*) is the 1st capturing group

([#]?[^\?]*) is the 2nd capturing group

(.*) is the 3rd capturing group

\1?parameter1=5273&\2\3

replaces the value for the 1st capturing group with "?parameter1=5273"

\2?parameter2=7465& replaces the value for the 2nd capturing group with "?parameter2=7465&"

\3 inserts the value matching the 3rd capturing group

^[\s\S]{50,}$

I hope this article helped you learn a few cool tricks you can do with Regular Expressions.

You can test your Regex on this website: https://regex101.com/. Note that you should select “Golang” from the left menu.