A regular expression (regex) is a sequence of characters that define a search pattern. Here’s how to write regular expressions:

1. Start by understanding the special characters used in regex, such as “.”, “*”, “+”, “?”, and more.
2. Choose a programming language or tool that supports regex, such as Python, Perl, or grep.
3. Write your pattern using the special characters and literal characters.
4. Use the appropriate function or method to search for the pattern in a string.