This lesson doesn’t teach anything new — it’s a reference you can scan in a few seconds whenever you’re writing a pattern and can’t quite remember the syntax for a lookbehind or a lazy quantifier. Everything here was covered in earlier lessons; this is the condensed version.
Anchors and boundaries
| Token | Meaning |
|---|---|
^ |
Start of string (or start of line, with the m flag) |
$ |
End of string (or end of line, with the m flag) |
\b |
Word boundary — between a word character and a non-word character |
\B |
Not a word boundary |
Character classes
| Token | Meaning |
|---|---|
. |
Any character except a line break |
\d |
A digit (0-9) |
\D |
Any non-digit |
\w |
A word character (letters, digits, underscore) |
\W |
Any non-word character |
\s |
A whitespace character (space, tab, newline) |
\S |
Any non-whitespace character |
[abc] |
Any one of a, b, or c |
[^abc] |
Any character except a, b, or c |
[a-z] |
Any character in the range a through z |
Quantifiers
| Token | Meaning |
|---|---|
* |
Zero or more (greedy) |
+ |
One or more (greedy) |
? |
Zero or one (greedy); also marks a quantifier as lazy when placed after one |
{n} |
Exactly n times |
{n,} |
n or more times |
{n,m} |
Between n and m times |
*?, +?, ?? |
Lazy versions — match as little as possible |
"<a><b>".match(/<.+>/); // greedy: "<a><b>"
"<a><b>".match(/<.+?>/); // lazy: "<a>"Groups
| Token | Meaning |
|---|---|
(...) |
Capturing group — remembers the matched text by number |
(?:...) |
Non-capturing group — groups for repetition/alternation without recording a capture |
(?<name>...) |
Named capturing group — access via match.groups.name |
| |
Alternation — “this or that” |
\1, \2 |
Backreference to a numbered group’s matched text |
\k<name> |
Backreference to a named group’s matched text |
Lookaround
| Token | Meaning |
|---|---|
(?=...) |
Positive lookahead — must be followed by this, not consumed |
(?!...) |
Negative lookahead — must NOT be followed by this |
(?<=...) |
Positive lookbehind — must be preceded by this, not consumed |
(?<!...) |
Negative lookbehind — must NOT be preceded by this |
"$100".match(/(?<=\$)\d+/); // "100" — the $ isn't part of the match
"100px".match(/\d+(?=px)/); // "100" — the px isn't part of the match
"foo1".match(/foo(?!bar)/); // "foo" — matches, next chars aren't "bar"Flags
| Flag | Name | Effect |
|---|---|---|
g |
Global | Find all matches, not just the first |
i |
Case-insensitive | Ignore letter case |
m |
Multiline | ^/$ match the start/end of each line, not just the whole string |
s |
Dot-all | . also matches line breaks |
u |
Unicode | Treats the pattern and string as full Unicode code points |
"Cat cat CAT".match(/cat/gi); // ["Cat", "cat", "CAT"]
"line1\nline2".match(/^line/gm); // ["line", "line"]Tip
This page is a quick recap, not a replacement for practice. For a searchable, copyable, always-up-to-date version of this same material, check out the full interactive cheat sheet — it’s designed to stay open in a tab while you write real patterns.
Try it yourself
Using only the tables above, write a pattern (without looking back at earlier lessons) that matches a hex color code like #a1b2c3 or #FFF — either 3 or 6 hex digits after a #, anchored so it matches the whole string.
What should happen
One correct answer: ^#(?:[0-9a-fA-F]{3}){1,2}$ doesn’t quite work because it would also accept 3-digit or 6-digit only in multiples of 3, not exactly 3 or 6 with nothing in between — the cleaner form is ^#(?:[0-9a-fA-F]{6}|[0-9a-fA-F]{3})$, an alternation between “exactly 6 hex digits” and “exactly 3 hex digits,” anchored on both ends so nothing shorter or longer sneaks through. This exercise is really testing whether you reach for alternation (|) instead of trying to force a single quantifier to express “3 or 6.”
What’s next
That’s the full course — from “what is a regex” all the way through catastrophic backtracking and production-ready best practices. From here, the best next step is practice: take a real problem from your own work and reach for this cheat sheet, or the interactive version, whenever you need a reminder.