Regular Expressions

What is a regular expression

A regular expression (or "regex") is a sequence of characters that is used to match a longer list of characters.

For example, this is a regular expression:
[df]og
which matches the words dog and fog.

You can use different combination of symbols in order to match different sets of characters.

"match" means "there is/are one or more matches in the text".

For example, let's take this text:
"There's a dog running in the fog."

regex 1:
dog
matches: 1
This regex matches the text.

regex 2
[df]og
matches: 2
This regex matches the text.

regex 3
rain
matches: 0
This regex doesn't match the text.

Square brackets are used whenever you want to establish an "OR" relation between what's inside them.
For example
[ab]
matches "a" and "b".

This regex:
file[01234]
will match