A regex looks like `/^[a-z]+@[a-z]+\.[a-z]+$/` and reads as 'one or more lowercase letters, then @, then more letters, then a dot, then more letters'.
Every modern language and editor supports them. The syntax is mostly shared, with small dialect differences (JavaScript, PCRE, .NET, POSIX).
Reach for regex when text follows a clear pattern. Don't use it for HTML (use a parser) or anything recursive (use a real grammar).