🎰 Regular Expressions (RegEx)

Various useful tools and resources for working with regular expressions.

Specifications

Engine / Flavor	Origin / Common Use	Lookahead	Lookbehind	Backreferences	Named Groups	Unicode \p{}	Non-greedy Quantifiers	Guarantees Linear Time	Notable Traits
POSIX (BRE/ERE)	grep, sed, awk	❌	❌	✅ (BRE limited)	❌	Partial	❌	✅ (deterministic)	Classic Unix tools, minimal
PCRE	PHP, Perl, Apache	✅	✅	✅	✅	✅	✅	❌	De facto standard, very expressive
Python re	Python	✅	✅ (fixed width)	✅	✅ (?P<>)	Partial (full via regex lib)	✅	❌	PCRE-like with some constraints
JavaScript (ECMAScript)	Web, Node.js	✅	✅ (since ES2018)	✅	✅ (?<name>)	✅ (\p{} in ES2018+)	✅	❌	Minimal but modern
google/re20 (0)	Go, Google infra	✅	❌	❌	✅	✅	✅	✅	No backtracking, ultra fast
Java java.util.regex	JVM ecosystem	✅	✅ (fixed width)	✅	✅	Partial	✅	❌	PCRE-ish, stable
.NET	C#, F#, etc.	✅	✅	✅	✅	✅	✅	❌	Very feature-rich, balancing groups
Rust regex	Rust crate	✅	❌	❌	✅	✅	✅	✅	Like RE2, safe & Unicode-first
PostgreSQL	SQL	✅ (limited)	❌	✅	❌	Partial	❌	❌	POSIX-based
Oniguruma	Ruby	✅	✅	✅	✅	✅	✅	❌	Advanced Unicode and named groups

💡 Tips

• Think about you want to "match" or "search"
• For matching, use ^ and $ to match the beginning and end of the string
  • Be careful with multi-line strings

Tools

• regex101
• Debuggex
• RegExr
• Rubular

Characters

Character	RegEx	Notes
Lowercase letter	[a-z]
Uppercase letter	[A-Z]
Letter	[a-zA-Z]
Digit	\d	Equal to [0-9]
Non-digit	\D
Whitespace	\s	Could be space, tab, etc.
Non-whitespace	\S
Word character	\w	Equal to [a-zA-Z0-9_]
Non-word	\W
Word boundary	\b
Non-boundary	\B

Quantifiers

note

In possessive match mode, the regex engine will not backtrack to find a match. This is useful when you know that there is only one possible match. This can be more efficient than greedy or reluctant matching, but more prone to not finding a match.

Quantifiers	RegEx	Notes
More or 0	*	Greedy Match (Prefer more)
More or 1	+	Greedy Match (Prefer more)
1 or 0	?	Greedy Match (Prefer one)
0 or more	*?	Reluctant Match (Prefer 0)
1 or more	+?	Reluctant Match (Prefer 1)
0 or 1	??	Reluctant Match (Prefer 0)
0 or more	*+	Possessive Match
1 or more	++	Possessive Match
1 or 0	?+	Possessive Match
Exactly n	{n}, {n}+, {n}?
At least n	{n,}	Greedy Match (Prefer more)
Between n and m	{n,m}	Greedy Match (Prefer more)
At least n	{n,}?	Reluctant Match (Prefer less)
Between n and m	{n,m}?	Reluctant Match (Prefer less)
At least n	{n,}+	Possessive Match
Between n and m	{n,m}+	Possessive Match

Snippets

File Name

This takes more than RegEx to fully validate, see String Sanitizing

Email

TODO

URL

TODO

Private IP Address¹

Start	End	CIDR
127.0.0.0	127.255.255.255	127.0.0.0/8
10.0.0.0	10.255.255.255	10.0.0.0/8
172.16.0.0	172. 31.255.255	172.16.0.0/12
192.168.0.0	192.168.255.255	192.168.0.0/16

(^127\.)|
(^10\.)|
(^172\.1[6-9]\.)|(^172\.2[0-9]\.)|(^172\.3[0-1]\.)|
(^192\.168\.)

Base64 encoded string

^[a-zA-Z0-9+/]*={0,2}$

URL-safe slug

• Have at least one character
• Contains only letters, numbers, hyphens, and underscores
• Does not start or end with a hyphen or underscore
• Does not have consecutive hyphens or underscores

^[a-zA-Z0-9]+([-_][a-zA-Z0-9]+)*$

Semantic Version

^(\d+)\.(\d+)\.(\d+)$

Chinese Characters

[\u4e00-\u9fa5]

🇹🇼 Taiwan Address

This is a WIP, might not be perfect.

^[\u4e00-\u9fa5]+?(縣|市)[\u4e00-\u9fa5]+?(鄉|鎮|市|區)[\u4e00-\u9fa5]+?(路|街|大道)([\u4e00-\u9fa5]+?段)?(\d+?巷)?(\d+?弄)?[\d\-、]+?號(.+?棟)?((B?[\d]+?|G|L)樓)?(之\d+?)?$

🇹🇼 Taiwan Address with Village

^[\u4e00-\u9fa5]+?(縣|市)[\u4e00-\u9fa5]+?(鄉|鎮|市|區)([\u4e00-\u9fa5]+?(村|里))?(\d+?鄰)?[\u4e00-\u9fa5]+?(路|街|大道)([\u4e00-\u9fa5]+?段)?(\d+?巷)?(\d+?弄)?[\d\-、]+?號(.+?棟)?((B?[\d]+?|G|L)樓)?(之\d+?)?$

References

• ICU Regular Expressions
• i Hate Regex

Footnotes

1. https://stackoverflow.com/a/2814102/10325430 ↩