Vol. I · A Field Guide For the working programmer Edition MMXXVI

The Regex
Cheat·sheet

Patterns, Anchors & Escapes — at a glance

Verified
/.*?/ Pattern · Match
§ Prefatory
Note

A regular expression is a tiny language for describing the shape of text. This sheet collects the pieces you will reach for most often — what to match, how much of it, and where. Examples assume the common /pattern/flags notation; small dialect differences exist between engines.

Contents

§ 01 · Chapter I

Character Classes

What kind of character to match. The vocabulary of single positions.

.
Any character
Matches any single character except a newline (unless the s flag is set).
exc.t → cat, cot, c7t
\d
Digit
A single digit, equivalent to [0-9].
ex\d\d → age 42
\D
Non-digit
Any character that is not a digit.
ex\D+ → price: $42
\w
Word character
A letter, digit, or underscore — equivalent to [A-Za-z0-9_].
ex\w+ → hello_world
\W
Non-word character
Anything other than a letter, digit, or underscore.
ex\W → hi!
\s
Whitespace
A space, tab, newline, or other whitespace character.
exa\sb → a b
\S
Non-whitespace
Any character that isn't whitespace.
ex\S+ → hello world
[abc]
Character set
Any one of the listed characters.
ex[aeiou] → cat
[^abc]
Negated set
Any character not in the list. The ^ inverts the class.
ex[^0-9] → abc!
[a-z]
Character range
A range of characters by code point. Combine ranges freely.
ex[A-Za-z0-9] → Hello42
§ 02 · Chapter II

Quantifiers

How many times the preceding piece should occur. Greedy by default; add ? to make them lazy.

*
Zero or more
Matches the previous item any number of times, including none.
exab* → a, ab, abbbb
+
One or more
Matches the previous item at least once.
ex\d+ → page 42
?
Optional
Zero or one — makes the preceding piece optional.
excolou?r → color, colour
{n}
Exactly n
Exactly n occurrences of the preceding item.
ex\d{4} → year 2026
{n,}
At least n
n or more occurrences.
ex\d{3,} → 12345
{n,m}
Between n and m
A range of repetitions, inclusive on both ends.
ex\d{2,4} → 42, 2026
*? +?
Lazy (non-greedy)
Adding ? to a quantifier makes it match as little as possible.
ex<.*?> → <b>hi</b>
*+ ++
Possessive
Matches greedily and never backtracks. Not supported in all engines (e.g. JavaScript).
ex\d++ → matches digits, won't release
§ 03 · Chapter III

Anchors & Boundaries

Anchors match positions, not characters. They specify where in the string a match must occur.

^
Start of string
Matches the beginning of the input. With the m flag, matches the start of each line.
ex^Hello → Hello world
$
End of string
Matches the end of the input (or end of each line with m).
exend$ → the end
\b
Word boundary
Position between a word character and a non-word character. No characters consumed.
ex\bcat\b → cat, not category
\B
Not a word boundary
The opposite of \b — inside a word, or between two non-word characters.
ex\Bcat → concatenate
\A
Absolute start
Start of input only — never affected by the m flag. Not in JavaScript.
ex\Aabc → abcdef
\z
Absolute end
End of input only. Many engines also accept \Z (end, but allows trailing newline).
exend\z → the end
§ 04 · Chapter IV

Groups & Alternation

Group pieces, choose between alternatives, and refer back to captured text.

(abc)
Capturing group
Groups expressions and saves the matched text for later reference.
ex(\d{3})-\d{4} → 555-1234
(?:abc)
Non-capturing group
Groups without saving — useful when you only need structure, not the capture.
ex(?:ab)+ → ababab
a|b
Alternation
Match either the left or right alternative. Lowest precedence — usually wrapped in a group.
excat|dog → cat, dog
(?<n>..)
Named group
Capture under a name, accessed via match.groups.n or similar.
ex(?<year>\d{4}) → 2026
\1 \2
Backreference
Reuses the text matched by an earlier capturing group, by number.
ex(\w+) \1 → the the
\k<n>
Named backreference
Reuses a named capture by name rather than position.
ex(?<c>\w)\k<c> → ll in hello
§ 05 · Chapter V

Escape Sequences

The backslash has two jobs: strip special meaning from metacharacters, and grant it to ordinary letters.

\.
Literal period
A real . rather than the "any character" metacharacter.
ex\d+\.\d+ → 3.14
\\
Literal backslash
A single backslash character in the input.
exC:\\\\ → C:\
\( \)
Literal parens
Match real parentheses instead of starting a group.
ex\(\d+\) → (42)
\[ \]
Literal brackets
Match real square brackets.
ex\[\w+\] → [note]
\? \* \+
Literal quantifiers
Escape these to match them as ordinary punctuation.
exare\? → are?
\|
Literal pipe
A vertical bar character, not alternation.
exa\|b → a|b
\n
Newline
Line-feed character (ASCII 10).
exline\nline → multi-line break
\t
Tab
Horizontal tab character (ASCII 9).
exa\tb → a→b
\r
Carriage return
ASCII 13. Often paired with \n on Windows line endings.
ex\r\n → Windows EOL
\xFF
Hex code
A character given by a two-digit hexadecimal value.
ex\x41 → A
\uFFFF
Unicode code point
A Unicode character given by a four-digit hex code.
ex\u00e9 → é
\0
Null character
The null byte (ASCII 0).
exrare outside binary contexts
§ 06 · Chapter VI

Flags

Modifiers that change how the entire pattern is interpreted. Written after the closing slash.

i
Case insensitive
Treats upper and lower case as equivalent.
ex/hello/i → HELLO, Hello
g
Global
Find every match, not just the first.
ex/a/g → banana
m
Multiline
Makes ^ and $ match start and end of each line.
ex/^foo/m → line-by-line
s
Dotall
Lets . match newlines too.
ex/a.b/s → a\nb
u
Unicode
Enables proper Unicode handling and \p{...} properties.
ex/\p{Emoji}/u → 🙂
x
Extended (free-spacing)
Ignores whitespace in the pattern, allowing comments. Not in JavaScript.
ex/\d+ # digits/x
§ 07 · Chapter VII

Lookarounds

Zero-width assertions. They check what's beside the current position without consuming any characters.

(?=..)
Positive lookahead
The following text must match the inner pattern, but isn't part of the match.
ex\d+(?=px) → 42px
(?!..)
Negative lookahead
The following text must not match.
ex\d+(?!px) → 42em
(?<=..)
Positive lookbehind
The preceding text must match the inner pattern.
ex(?<=\$)\d+ → $42
(?<!..)
Negative lookbehind
The preceding text must not match.
ex(?<!\$)\d+ → qty 42
§ 08 · Chapter VIII

Common Recipes

Useful starting points. Tweak to taste — every dataset has its quirks.

№ I Email address (simplified)
/\b[\w.+-]+@[\w-]+\.[\w.-]+\b/i
№ II URL
/https?:\/\/[\w.-]+(?:\/[\w./?&=#%-]*)?/gi
№ III Hex color code
/^#[0-9a-f]{3}([0-9a-f]{3})?$/i
№ IV ISO date · YYYY-MM-DD
/\b\d{4}-\d{2}-\d{2}\b/
№ V 24-hour time
/^(?:[01]\d|2[0-3]):[0-5]\d$/
№ VI Phone · (555) 123-4567
/\(\d{3}\) \d{3}-\d{4}/
№ VII IPv4 address (loose)
/\b\d{1,3}(?:\.\d{1,3}){3}\b/
№ VIII Duplicate words
/\b(\w+)\s+\1\b/gi
№ IX Trim whitespace
/^\s+|\s+$/g
№ X Markdown link
/\[([^\]]+)\]\(([^)]+)\)/g
§ 09 · Chapter IX

The Laboratory

Try a pattern against text. Matches are highlighted in real time.

Pattern
/ /
Subject
Results
— matches