Match the start of a string or line.

^Hello

Match the end of a string or line.

world$

Match a word boundary.

\bcat\b

Match when not on a word boundary.

\Bing\B

Repeat the previous token zero or more times.

go*

Repeat the previous token one or more times.

go+

Make the previous token optional.

colou?r

Repeat between n and m times.

\d{2,4}

Create a capturing group.

(cat|dog)

Create a non-capturing group.

(?:https?)

Create a named capture group.

(?<year>\d{4})

Reference the first capture group.

(\w+)\s+\1

Match any character except newlines by default.

a.c

Match a digit or non-digit.

\d{5}

Match a word or non-word character.

\w+

Match from or exclude a custom set.

[A-F0-9]+

Positive lookahead.

\w+(?=@)

Negative lookahead.

foo(?!bar)

Positive lookbehind.

(?<=\$)\d+

Negative lookbehind.

(?<!-)\d+