Related
This question's answers are a community effort. Edit existing answers to improve this post. It is not currently accepting new answers or interactions.
What is this?
This is a collection of common Q&A. This is also a Community Wiki, so everyone is invited to participate in maintaining it.
Why is this?
regex is suffering from give me ze code type of questions and poor answers with no explanation. This reference is meant to provide links to quality Q&A.
What's the scope?
This reference is meant for the following languages: php, perl, javascript, python, ruby, java, .net.
This might be too broad, but these languages share the same syntax. For specific features there's the tag of the language behind it, example:
What are regular expression Balancing Groups? .net
The Stack Overflow Regular Expressions FAQ
See also a lot of general hints and useful links at the regex tag details page.
Online tutorials
RegexOne ↪
Regular Expressions Info ↪
Quantifiers
Zero-or-more: *:greedy, *?:reluctant, *+:possessive
One-or-more: +:greedy, +?:reluctant, ++:possessive
?:optional (zero-or-one)
Min/max ranges (all inclusive): {n,m}:between n & m, {n,}:n-or-more, {n}:exactly n
Differences between greedy, reluctant (a.k.a. "lazy", "ungreedy") and possessive quantifier:
Greedy vs. Reluctant vs. Possessive Quantifiers
In-depth discussion on the differences between greedy versus non-greedy
What's the difference between {n} and {n}?
Can someone explain Possessive Quantifiers to me? php, perl, java, ruby
Emulating possessive quantifiers .net
Non-Stack Overflow references: From Oracle, regular-expressions.info
Character Classes
What is the difference between square brackets and parentheses?
[...]: any one character, [^...]: negated/any character but
[^] matches any one character including newlines javascript
[\w-[\d]] / [a-z-[qz]]: set subtraction .net, xml-schema, xpath, JGSoft
[\w&&[^\d]]: set intersection java, ruby 1.9+
[[:alpha:]]:POSIX character classes
[[:<:]] and [[:>:]] Word boundaries
Why do [^\\D2], [^[^0-9]2], [^2[^0-9]] get different results in Java? java
Shorthand:
Digit: \d:digit, \D:non-digit
Word character (Letter, digit, underscore): \w:word character, \W:non-word character
Whitespace: \s:whitespace, \S:non-whitespace
Unicode categories (\p{L}, \P{L}, etc.)
Escape Sequences
Horizontal whitespace: \h:space-or-tab, \t:tab
Newlines:
\r, \n:carriage return and line feed
\R:generic newline php java-8
Negated whitespace sequences: \H:Non horizontal whitespace character, \V:Non vertical whitespace character, \N:Non line feed character pcre php5 java-8
Other: \v:vertical tab, \e:the escape character
Anchors
anchor
matches
flavors
^
Start of string
Common*
^
Start of line
Commonm
$
End of line
Commonm
$
End of text
Common* except javascript
$
Very end of string
javascript*, phpD
\A
Start of string
Common except javascript
\Z
End of text
Common except javascript python
\Z
Very end of string
python
\z
Very end of string
Common except javascript python
\b
Word boundary
Common
\B
Not a word boundary
Common
\G
End of previous match
Common except javascript, python
Term
Definition
Start of string
At the very start of the string.
Start of line
At the very start of the string, andafter a non-terminal line terminator.
Very end of string
At the very end of the string.
End of text
At the very end of the string, andat a terminal line terminator.
End of line
At the very end of the string, andat a line terminator.
Word boundary
At a word character not preceded by a word character, andat a non-word character not preceded by a non-word character.
End of previous match
At a previously set position, usually where a previous match ended.At the very start of the string if no position was set.
"Common" refers to the following: icu java javascript .net objective-c pcre perl php python swift ruby
* Default |
m Multi-line mode. |
D Dollar end only mode.
Groups
(...):capture group, (?:):non-capture group
Why is my repeating capturing group only capturing the last match?
\1:backreference and capture-group reference, $1:capture group reference
What's the meaning of a number after a backslash in a regular expression?
\g<1>123:How to follow a numbered capture group, such as \1, with a number?: python
What does a subpattern (?i:regex) mean?
What does the 'P' in (?P<group_name>regexp) mean?
(?>):atomic group or independent group, (?|):branch reset
Equivalent of branch reset in .NET/C# .net
Named capture groups:
General named capturing group reference at regular-expressions.info
java: (?<groupname>regex): Overview and naming rules (Non-Stack Overflow links)
Other languages: (?P<groupname>regex) python, (?<groupname>regex) .net, (?<groupname>regex) perl, (?P<groupname>regex) and (?<groupname>regex) php
Lookarounds
Lookaheads: (?=...):positive, (?!...):negative
Lookbehinds: (?<=...):positive, (?<!...):negative
Lookbehind limits in:
Lookbehinds need to be constant-length php, perl, python, ruby
Lookarounds of limited length {0,n} java
Variable length lookbehinds are allowed .net
Lookbehind alternatives:
Using \K php, perl (Flavors that support \K)
Alternative regex module for Python python
The hacky way
JavaScript negative lookbehind equivalents External link
Modifiers
flag
modifier
flavors
a
ASCII
python
c
current position
perl
e
expression
php perl
g
global
most
i
case-insensitive
most
m
multiline
php perl python javascript .net java
m
(non)multiline
ruby
o
once
perl ruby
r
non-destructive
perl
S
study
php
s
single line
ruby
U
ungreedy
php r
u
unicode
most
x
whitespace-extended
most
y
sticky ↪
javascript
How to convert preg_replace e to preg_replace_callback?
What are inline modifiers?
What is '?-mix' in a Ruby Regular Expression
Other:
|:alternation (OR) operator, .:any character, [.]:literal dot character
What special characters must be escaped?
Control verbs (php and perl): (*PRUNE), (*SKIP), (*FAIL) and (*F)
php only: (*BSR_ANYCRLF)
Recursion (php and perl): (?R), (?0) and (?1), (?-1), (?&groupname)
Common Tasks
Get a string between two curly braces: {...}
Match (or replace) a pattern except in situations s1, s2, s3...
How do I find all YouTube video ids in a string using a regex?
Validation:
Internet: email addresses, URLs (host/port: regex and non-regex alternatives), passwords
Numeric: a number, min-max ranges (such as 1-31), phone numbers, date
Parsing HTML with regex: See "General Information > When not to use Regex"
Advanced Regex-Fu
Strings and numbers:
Regular expression to match a line that doesn't contain a word
How does this PCRE pattern detect palindromes?
Match strings whose length is a fourth power
How does this regex find triangular numbers?
How to determine if a number is a prime with regex?
How to match the middle character in a string with regex?
Other:
How can we match a^n b^n?
Match nested brackets
Using a recursive pattern php, perl
Using balancing groups .net
“Vertical” regex matching in an ASCII “image”
List of highly up-voted regex questions on Code Golf
How to make two quantifiers repeat the same number of times?
An impossible-to-match regular expression: (?!a)a
Match/delete/replace this except in contexts A, B and C
Match nested brackets with regex without using recursion or balancing groups?
Flavor-Specific Information
(Except for those marked with *, this section contains non-Stack Overflow links.)
Java
Official documentation: Pattern Javadoc ↪, Oracle's regular expressions tutorial ↪
The differences between functions in java.util.regex.Matcher:
matches()): The match must be anchored to both input-start and -end
find()): A match may be anywhere in the input string (substrings)
lookingAt(): The match must be anchored to input-start only
(For anchors in general, see the section "Anchors")
The only java.lang.String functions that accept regular expressions: matches(s), replaceAll(s,s), replaceFirst(s,s), split(s), split(s,i)
*An (opinionated and) detailed discussion of the disadvantages of and missing features in java.util.regex
.NET
How to read a .NET regex with look-ahead, look-behind, capturing groups and back-references mixed together?
Official documentation:
Boost regex engine: General syntax, Perl syntax (used by TextPad, Sublime Text, UltraEdit, ...???)
JavaScript general info and RegExp object
.NET MySQL Oracle Perl5 version 18.2
PHP: pattern syntax, preg_match
Python: Regular expression operations, search vs match, how-to
Rust: crate regex, struct regex::Regex
Splunk: regex terminology and syntax and regex command
Tcl: regex syntax, manpage, regexp command
Visual Studio Find and Replace
General information
(Links marked with * are non-Stack Overflow links.)
Other general documentation resources: Learning Regular Expressions, *Regular-expressions.info, *Wikipedia entry, *RexEgg, Open-Directory Project
DFA versus NFA
Generating Strings matching regex
Books: Jeffrey Friedl's Mastering Regular Expressions
When to not use regular expressions:
Some people, when confronted with a problem, think "I know, I'll use regular expressions." Now they have two problems. (blog post written by Stack Overflow's founder)*
Do not use regex to parse HTML:
Don't. Please, just don't
Well, maybe...if you're really determined (other answers in this question are also good)
Examples of regex that can cause regex engine to fail
Why does this regular expression kill the Java regex engine?
Tools: Testers and Explainers
(This section contains non-Stack Overflow links.)
Online (* includes replacement tester, + includes split tester):
Debuggex (Also has a repository of useful regexes) javascript, python, pcre
*Regular Expressions 101 php, pcre, python, javascript, java
Regex Pal, regular-expressions.info javascript
Rubular ruby RegExr Regex Hero dotnet
*+ regexstorm.net .net
*RegexPlanet: Java java, Go go, Haskell haskell, JavaScript javascript, .NET dotnet, Perl perl php PCRE php, Python python, Ruby ruby, XRegExp xregexp
freeformatter.com xregexp
*+regex.larsolavtorvik.com php PCRE and POSIX, javascript
Offline:
Microsoft Windows: RegexBuddy (analysis), RegexMagic (creation), Expresso (analysis, creation, free)
MySQL 8.0: Various syntax changes were made. Note especially the doubling of backslashes in some contexts. (This Answer need further editing to reflect the differences.)
There are times when you need to input modified variables with diacritical marks, or superscripts.
Seems like declare_index_properties allows doing it at the stage of display print.
But it is neither simple, nor very useful in formulas.
is there a simple way of adding hats, umlauts, and ', "strokes on top of a symbol, making it distinguishable from the symbol without such mark both to interpreter and to human eye?
Maxima doesn't have a notion of declaring a symbol to have diacritical marks or other combining marks on it. However, Maxima allows Unicode characters in symbol names if the underlying Lisp implementation allows Unicode; almost all of them allow Unicode. GCL is the only Lisp implementation, so far as I know, which doesn't handle Unicode correctly.
WxMaxima appears to allow Unicode characters to be input. At least, it worked that way when I tried some examples. Command-line Maxima allows Unicode if the terminal it is running in allows Unicode.
I think any Unicode character should be OK in a string. For symbols, any character which passes ALPHA-CHAR-P (a build-in Lisp function) can be part of a symbol name. Also, any character which is declared to be alphabetic (via declare("x", alphabetic) where x is the character in question) can be part of a symbol name.
I think wxMaxima has some capability to allow the user to select characters with diacritical marks from a menu; I haven't tried it. When I want to use Unicode characters, I end up just pasting them from a web page or something. I have used https://www.w3.org/2001/06/utf-8-test/UTF-8-demo.html as a source of characters in the past.
This question's answers are a community effort. Edit existing answers to improve this post. It is not currently accepting new answers or interactions.
What is this?
This is a collection of common Q&A. This is also a Community Wiki, so everyone is invited to participate in maintaining it.
Why is this?
regex is suffering from give me ze code type of questions and poor answers with no explanation. This reference is meant to provide links to quality Q&A.
What's the scope?
This reference is meant for the following languages: php, perl, javascript, python, ruby, java, .net.
This might be too broad, but these languages share the same syntax. For specific features there's the tag of the language behind it, example:
What are regular expression Balancing Groups? .net
The Stack Overflow Regular Expressions FAQ
See also a lot of general hints and useful links at the regex tag details page.
Online tutorials
RegexOne ↪
Regular Expressions Info ↪
Quantifiers
Zero-or-more: *:greedy, *?:reluctant, *+:possessive
One-or-more: +:greedy, +?:reluctant, ++:possessive
?:optional (zero-or-one)
Min/max ranges (all inclusive): {n,m}:between n & m, {n,}:n-or-more, {n}:exactly n
Differences between greedy, reluctant (a.k.a. "lazy", "ungreedy") and possessive quantifier:
Greedy vs. Reluctant vs. Possessive Quantifiers
In-depth discussion on the differences between greedy versus non-greedy
What's the difference between {n} and {n}?
Can someone explain Possessive Quantifiers to me? php, perl, java, ruby
Emulating possessive quantifiers .net
Non-Stack Overflow references: From Oracle, regular-expressions.info
Character Classes
What is the difference between square brackets and parentheses?
[...]: any one character, [^...]: negated/any character but
[^] matches any one character including newlines javascript
[\w-[\d]] / [a-z-[qz]]: set subtraction .net, xml-schema, xpath, JGSoft
[\w&&[^\d]]: set intersection java, ruby 1.9+
[[:alpha:]]:POSIX character classes
[[:<:]] and [[:>:]] Word boundaries
Why do [^\\D2], [^[^0-9]2], [^2[^0-9]] get different results in Java? java
Shorthand:
Digit: \d:digit, \D:non-digit
Word character (Letter, digit, underscore): \w:word character, \W:non-word character
Whitespace: \s:whitespace, \S:non-whitespace
Unicode categories (\p{L}, \P{L}, etc.)
Escape Sequences
Horizontal whitespace: \h:space-or-tab, \t:tab
Newlines:
\r, \n:carriage return and line feed
\R:generic newline php java-8
Negated whitespace sequences: \H:Non horizontal whitespace character, \V:Non vertical whitespace character, \N:Non line feed character pcre php5 java-8
Other: \v:vertical tab, \e:the escape character
Anchors
anchor
matches
flavors
^
Start of string
Common*
^
Start of line
Commonm
$
End of line
Commonm
$
End of text
Common* except javascript
$
Very end of string
javascript*, phpD
\A
Start of string
Common except javascript
\Z
End of text
Common except javascript python
\Z
Very end of string
python
\z
Very end of string
Common except javascript python
\b
Word boundary
Common
\B
Not a word boundary
Common
\G
End of previous match
Common except javascript, python
Term
Definition
Start of string
At the very start of the string.
Start of line
At the very start of the string, andafter a non-terminal line terminator.
Very end of string
At the very end of the string.
End of text
At the very end of the string, andat a terminal line terminator.
End of line
At the very end of the string, andat a line terminator.
Word boundary
At a word character not preceded by a word character, andat a non-word character not preceded by a non-word character.
End of previous match
At a previously set position, usually where a previous match ended.At the very start of the string if no position was set.
"Common" refers to the following: icu java javascript .net objective-c pcre perl php python swift ruby
* Default |
m Multi-line mode. |
D Dollar end only mode.
Groups
(...):capture group, (?:):non-capture group
Why is my repeating capturing group only capturing the last match?
\1:backreference and capture-group reference, $1:capture group reference
What's the meaning of a number after a backslash in a regular expression?
\g<1>123:How to follow a numbered capture group, such as \1, with a number?: python
What does a subpattern (?i:regex) mean?
What does the 'P' in (?P<group_name>regexp) mean?
(?>):atomic group or independent group, (?|):branch reset
Equivalent of branch reset in .NET/C# .net
Named capture groups:
General named capturing group reference at regular-expressions.info
java: (?<groupname>regex): Overview and naming rules (Non-Stack Overflow links)
Other languages: (?P<groupname>regex) python, (?<groupname>regex) .net, (?<groupname>regex) perl, (?P<groupname>regex) and (?<groupname>regex) php
Lookarounds
Lookaheads: (?=...):positive, (?!...):negative
Lookbehinds: (?<=...):positive, (?<!...):negative
Lookbehind limits in:
Lookbehinds need to be constant-length php, perl, python, ruby
Lookarounds of limited length {0,n} java
Variable length lookbehinds are allowed .net
Lookbehind alternatives:
Using \K php, perl (Flavors that support \K)
Alternative regex module for Python python
The hacky way
JavaScript negative lookbehind equivalents External link
Modifiers
flag
modifier
flavors
a
ASCII
python
c
current position
perl
e
expression
php perl
g
global
most
i
case-insensitive
most
m
multiline
php perl python javascript .net java
m
(non)multiline
ruby
o
once
perl ruby
r
non-destructive
perl
S
study
php
s
single line
ruby
U
ungreedy
php r
u
unicode
most
x
whitespace-extended
most
y
sticky ↪
javascript
How to convert preg_replace e to preg_replace_callback?
What are inline modifiers?
What is '?-mix' in a Ruby Regular Expression
Other:
|:alternation (OR) operator, .:any character, [.]:literal dot character
What special characters must be escaped?
Control verbs (php and perl): (*PRUNE), (*SKIP), (*FAIL) and (*F)
php only: (*BSR_ANYCRLF)
Recursion (php and perl): (?R), (?0) and (?1), (?-1), (?&groupname)
Common Tasks
Get a string between two curly braces: {...}
Match (or replace) a pattern except in situations s1, s2, s3...
How do I find all YouTube video ids in a string using a regex?
Validation:
Internet: email addresses, URLs (host/port: regex and non-regex alternatives), passwords
Numeric: a number, min-max ranges (such as 1-31), phone numbers, date
Parsing HTML with regex: See "General Information > When not to use Regex"
Advanced Regex-Fu
Strings and numbers:
Regular expression to match a line that doesn't contain a word
How does this PCRE pattern detect palindromes?
Match strings whose length is a fourth power
How does this regex find triangular numbers?
How to determine if a number is a prime with regex?
How to match the middle character in a string with regex?
Other:
How can we match a^n b^n?
Match nested brackets
Using a recursive pattern php, perl
Using balancing groups .net
“Vertical” regex matching in an ASCII “image”
List of highly up-voted regex questions on Code Golf
How to make two quantifiers repeat the same number of times?
An impossible-to-match regular expression: (?!a)a
Match/delete/replace this except in contexts A, B and C
Match nested brackets with regex without using recursion or balancing groups?
Flavor-Specific Information
(Except for those marked with *, this section contains non-Stack Overflow links.)
Java
Official documentation: Pattern Javadoc ↪, Oracle's regular expressions tutorial ↪
The differences between functions in java.util.regex.Matcher:
matches()): The match must be anchored to both input-start and -end
find()): A match may be anywhere in the input string (substrings)
lookingAt(): The match must be anchored to input-start only
(For anchors in general, see the section "Anchors")
The only java.lang.String functions that accept regular expressions: matches(s), replaceAll(s,s), replaceFirst(s,s), split(s), split(s,i)
*An (opinionated and) detailed discussion of the disadvantages of and missing features in java.util.regex
.NET
How to read a .NET regex with look-ahead, look-behind, capturing groups and back-references mixed together?
Official documentation:
Boost regex engine: General syntax, Perl syntax (used by TextPad, Sublime Text, UltraEdit, ...???)
JavaScript general info and RegExp object
.NET MySQL Oracle Perl5 version 18.2
PHP: pattern syntax, preg_match
Python: Regular expression operations, search vs match, how-to
Rust: crate regex, struct regex::Regex
Splunk: regex terminology and syntax and regex command
Tcl: regex syntax, manpage, regexp command
Visual Studio Find and Replace
General information
(Links marked with * are non-Stack Overflow links.)
Other general documentation resources: Learning Regular Expressions, *Regular-expressions.info, *Wikipedia entry, *RexEgg, Open-Directory Project
DFA versus NFA
Generating Strings matching regex
Books: Jeffrey Friedl's Mastering Regular Expressions
When to not use regular expressions:
Some people, when confronted with a problem, think "I know, I'll use regular expressions." Now they have two problems. (blog post written by Stack Overflow's founder)*
Do not use regex to parse HTML:
Don't. Please, just don't
Well, maybe...if you're really determined (other answers in this question are also good)
Examples of regex that can cause regex engine to fail
Why does this regular expression kill the Java regex engine?
Tools: Testers and Explainers
(This section contains non-Stack Overflow links.)
Online (* includes replacement tester, + includes split tester):
Debuggex (Also has a repository of useful regexes) javascript, python, pcre
*Regular Expressions 101 php, pcre, python, javascript, java
Regex Pal, regular-expressions.info javascript
Rubular ruby RegExr Regex Hero dotnet
*+ regexstorm.net .net
*RegexPlanet: Java java, Go go, Haskell haskell, JavaScript javascript, .NET dotnet, Perl perl php PCRE php, Python python, Ruby ruby, XRegExp xregexp
freeformatter.com xregexp
*+regex.larsolavtorvik.com php PCRE and POSIX, javascript
Offline:
Microsoft Windows: RegexBuddy (analysis), RegexMagic (creation), Expresso (analysis, creation, free)
MySQL 8.0: Various syntax changes were made. Note especially the doubling of backslashes in some contexts. (This Answer need further editing to reflect the differences.)
What is an unicode code for grave accent mark used to specify regex literal in Frege?
The character is called Acute Accent and the unicode for that is 00B4. In ubuntu, you can type that using Ctrl+Shift+u and then type 00B4 then space. However you don't really have to use that if your regex literal is more than one character in which case you can just use apostrophes.
Quoting the doc:
Regular expression literals have type Regex and are written:
´\b(foo|bar)\b´ -- string enclosed in grave accents
'\w+' -- string with length > 1 enclosed in apostrophes
The notation with the apostrophes has been introduced because many have a hard time entering a grave accent mark on their terminal. However, it is not possible to write a regular expressions with length 1 this way, because then the literal gets interpreted as Char literal. (One can write something like '(?:X)' for a Regex that matches a single 'X').
I need to create an XPath query to select a JCR node whose name contains a whitespace character.
For instance: /jcr:root/foo bar/
But that results in an invalid query.
How should whitespaces be encoded in an XPath query?
Try using something like this XPath query:
/jcr:root/foo_x0020_bar/
The JSR-170 (JCR 1.0) specification defines how XPath can be used to query a JCR repository, and even though JSR-283 (or JCR 2.0) deprecated XPath as a query language, many of the implementations still support XPath along with the other query languages (including the more powerful JCR-SQL2).
Now, regarding the rules for escaping characters in XPath, JSR-170 states the following in Section 6.6.4.9:
The names of elements and attributes (corresponding to nodes and properties, respectively) within an XPath statement must correspond to the form in which they (notionally) appear in the document view. This means that spaces (and any other non-XML characters) within names must be encoded according to the rules described in 6.4.3 Escaping of Names.
Section 6.4.3 defines how such characters are escaped in names:
The escape character is the underscore (“_”). Any invalid character is escaped as _xHHHH_, where HHHH is the four-digit hexadecimal UTF-16 code for the character. When producing escape sequences the implementation should use lowercase letters for the hex digits a-f. When unescaping, however, both upper and lowercase alphabetic hexadecimal characters must be recognized.
Although you didn't ask about it, you can easily do the same query in JCR-SQL2:
SELECT * FROM [nt:base] WHERE ISSAMENODE('/foo_x0020_bar')