Regex

Used for pattern matching in string. Common use cases include text validation and text search.

Regexes are enclosed within / / (this document doesn't use these enclosings in most regexes to keep it clean).

Some common ones

[abc] - a, b or c
[^abc] - any character except a, b, c
[a-z] - a to z
[A-Z] - A to Z
[a-zA-Z] - a to z, A to Z
[0-9] - 0 to 9

Flags

g

/a/ will only match the first occurence of a. /a/g will match all occurences of a throughout the string.

Quantifiers

[ ]? - occurs 0 or 1 times
[ ]+ - occurs 1 or more times
[ ]* - occurs 0 or more times
[ ]{n} - occurs n times
[ ]{n,} - occurs n or more times
[ ]{y,z} - occurs at least y times but less than z times

Metacharacters

\d - all digits ≍ [0-9]
\w - all "word" (alphanumeric) characters ≍ [a-zA-Z0-9]
\D - everything except digits ≍ [^0-9]
\W - everything except alphanumeric characters ≍ [^a-z]
. - any character except a newline

Escaping

Use \ to replace any special character.

Logical Operators

^ - Negation
| - Logical OR
() - to group your logic

Standards

Regular expressions (regex) are not defined by a single, universally accepted standard like an RFC. However, there are efforts to establish some level of consistency across different regex implementations.

Some regex standards are:

POSIX Basic Regular Expressions (BRE)
Extended Regular Expressions (ERE)
PCRE (Perl Compatible Regular Expressions)
I-Regexp (RFC 9485)

References