Repetition

Repetition is a powerful and ubiquitous regex feature. There are several ways to represent repetition in regex.

Making things optional

We can make parts of regex optional using the ? operator.

Here’s another example:

Here the s following http is optional.

We can also make capturing and non-capturing groups optional.

Zero or more

If we wish to match zero or more of a token, we can suffix it with *.

Our regex matches even an empty string "".

One or more

If we wish to match one or more of a token, we can suffix it with a +.

Exactly x times

If we wish to match a particular token exactly x times, we can suffix it with {x}. This is functionally identical to repeatedly copy-pasting the token x times.

Here’s an example that matches an uppercase six-character hex colour code.

Here, the token {6} applies to the character class [0-9A-F].

Between min and max times

If we wish to match a particular token between min and max (inclusive) times, we can suffix it with {min,max}.

At least x times

If we wish to match a particular token at least x times, we can suffix it with {x,}. Think of it as {min,max}, but without an upper bound.

A note on greediness

Regular expressions, by default, are greedy. They attempt to match as much as possible.

Suffixing a repetition operator (?, *, +, …) with a ?, one can make it “lazy”.

Here, this could also be achieved by using [^"] instead of . (as is best practice).

[…] Lazy will stop as soon as the condition is satisfied, but greedy means it will stop only once the condition is not satisfied any more

—Andrew S on StackOverflow

Examples

Bitcoin address

Youtube Video

We can adjust this to not match the last broken link using anchors, which we shall encounter soon.