Cheat Sheet

Test expressions interactively on regex101; it explains every token and supports the different flavors.

Example

How ^(\w+)@([\w.-]+)\.(\w{2,})$ matches ada@mail.example.org:

flowchart LR
  user["^(\w+)<br/>ada"] -->|"@"| domain["([\w.-]+)<br/>mail.example"]
  domain -->|"\."| tld["(\w{2,})$<br/>org"]

[\w.-]+ is greedy and first takes mail.example.org, then gives characters back until \.(\w{2,})$ can match the end.

Expression	Matches
`.`	Any character except a newline.
`\d`, `\D`	A digit, anything but a digit.
`\w`, `\W`	A letter, digit or underscore, anything else.
`\s`, `\S`	Whitespace (space, tab, newline), anything else.
`[abc]`	One of `a`, `b` or `c`.
`[^abc]`	Any character except `a`, `b` or `c`.
`[a-zA-Z]`	A letter in the given ranges.
`\n`, `\r`, `\t`	Newline, carriage return, tab.
`\.`	A literal dot. `\` escapes any special character.

Quantifiers are greedy: they match as much as possible. Add ? to make them lazy:

Text	Expression	Match
`<b>bold</b> text`	`<.+>`	`<b>bold</b>`
`<b>bold</b> text`	`<.+?>`	`<b>`

Expression	Matches
`^`, `$`	Start and end of the string (of each line with the `m` flag).
`\b`	A word boundary: `\bcat\b` matches `cat` but not `concat`.
`a\|b`	`a` or `b`.
`(abc)`	A capturing group, available as `\1` or `$1` in replacements.
`(?:abc)`	A group that doesn’t capture.
`(?<name>abc)`	A named group.
`a(?=b)`	`a` followed by `b`, without consuming `b` (lookahead).
`a(?!b)`	`a` not followed by `b`.
`(?<=b)a`	`a` preceded by `b` (lookbehind).

Flag	Effect
`g`	Global: finds all matches instead of stopping after the first.
`i`	Case-insensitive.
`m`	Multiline: `^` and `$` match at every line break.
`s`	Dot-all: `.` also matches newlines.