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Regular Expression Reference: Mode Modifiers

Mode modifier syntax consists of two elements that differ among regex flavors. Parentheses and a question mark are used to add the modifier to the regex. Depending on its position in the regex and the regex flavor it may affect the whole regex or part of it. If a flavor supports at least one modifier syntax, then it will also support one or more letters that can be used inside the modifier to toggle specific modes. If it doesn’t then “n/a” is indicated for all letters for that flavor.

If a flavor supports more than one modifier syntax then it supports the same options for all syntax, perhaps with the exception of -, x, and the locale settings which may not be supported in the middle of the regex. So the table below has one entry for each letter that applies to all modifier syntax, but separate entries for - and x depending on the modifier syntax.

If a flavor supports mode modifiers but does not support a particular letter then that letter is indicated as “no”. That does not mean that the flavor doesn’t have this mode at all. The flavor may still have the mode, but no option to turn it off. Modes are also not necessarily off by default. For example, in most regex flavors, ^ and $ match at the start and end of the string only by default. But the Just Great Software applications and Ruby, they match at the start and end of each line by default. In the JGsoft applications, you can turn off this mode with (?-m) while in Ruby you cannot turn off this mode at all. (?-m) affects the dot rather than the anchors in Ruby.

The table below only indicates whether each flavor supports a particular letter to toggle a particular mode. It does not indicate the defaults.

Feature	Syntax	Description	Example	JGsoft	Python	JavaScript	VBScript	XRegExp	.NET	Java	ICU	RE2	Perl	PCRE	PCRE2	PHP	Delphi	R	Ruby	std::regex	Boost	Tcl	POSIX	GNU	Oracle	XML	XPath
Mode modifier	`(?letters)` at the start of the regex	A mode modifier at the start of the regex affects the whole regex and overrides any options set outside the regex.	`(?i)a` matches `a` and `A`.	YES	YES	no	no	YES	YES	YES	YES	default	YES	YES	YES	YES	YES	YES	YES	no	ECMA	YES	no	no	no	no	no
Mode modifier	`(?letters)` in the middle of the regex	A mode modifier in the middle of the regex affects the whole regex and overrides any options set outside the regex.	`te(?i)st` matches `test`, `teST`, `TEst` and `TEST`.	no	2.4–3.6	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no
Mode modifier	`(?letters)` in the middle of the regex	A mode modifier in the middle of the regex affects only the part of the regex to the right of the modifier. If the modifier is used inside a group, it only affects the part of the regex inside that group to the right of the modifier. If the regex or group uses alternation, all alternatives to the right of the modifier are affected.	`te(?i)st` matches `test` and `teST` but not `TEst` or `TEST`.	YES	no	no	no	no	YES	YES	YES	default	YES	YES	YES	YES	YES	YES	YES	no	ECMA	no	no	no	no	no	no
Mode modifier	`(?letters)` followed by alternation	A mode modifier in the middle of the regex affects alternatives to the right of the modifier up until the end of the group that contains the modifier or until the end of the regex.	`te(?i)st\|two` matches `TWO`.	YES	n/a	n/a	n/a	n/a	YES	YES	YES	default	YES	YES	YES	YES	YES	YES	1.9	n/a	ECMA	n/a	n/a	n/a	n/a	n/a	n/a
Modifier group	`(?letters:regex)`	Non-capturing group with modifiers that affect only the part of the regex inside the group.	`te(?i:st)` matches `test` and `teST` but not `TEst` or `TEST`.	YES	3.6	YES	no	YES	YES	YES	YES	default	YES	YES	YES	YES	YES	YES	YES	no	ECMA	no	no	no	no	no	no
Negative modifier	`(?on-off)` at the start of the regex	Modifier letters (if any) before the hyphen are turned on, while modifier letters after the hyphen are turned off.	`(?-i)test` matches `test` but not `Test`.	YES	no	n/a	n/a	no	YES	YES	YES	default	YES	YES	YES	YES	YES	YES	YES	n/a	ECMA	no	n/a	n/a	n/a	n/a	n/a
Negative modifier	`(?on-off)` in the middle of the regex and/or `(?on-off:regex)`	Modifier letters (if any) before the hyphen are turned on, while modifier letters after the hyphen are turned off.	`(?i)te(?-i)st` matches `test` and `TEst` but not `teST` or `TEST`.	YES	3.6	YES	n/a	YES	YES	YES	YES	default	YES	YES	YES	YES	YES	YES	YES	n/a	ECMA	n/a	n/a	n/a	n/a	n/a	n/a
Reset modifiers	`(?^)`	Turn off all options. The caret can be followed by modifier letters to turn some options back on.	`(?i)te(?^)st` matches `test` and `TEst` but not `teST` or `TEST`.	no	no	no	n/a	no	no	no	no	no	YES	no	10.32	7.3.0	no	4.0.0	no	n/a	no	no	n/a	n/a	n/a	n/a	n/a
Free-spacing	`(?x)` at the start of the regex	Turn on free-spacing mode to ignore whitespace between regex tokens and allow # comments.	`(?x)a#b` matches `a`	YES	YES	n/a	n/a	YES	YES	YES	YES	no	YES	YES	YES	YES	YES	YES	YES	n/a	ECMA	YES	n/a	n/a	n/a	n/a	n/a
Free-spacing	`(?x)` in the middle of the regex or a group `(?x:regex)`	Turn on free-spacing mode to ignore whitespace between regex tokens and allow # comments after previously ignoring whitespace.	`(?x:a#b )` matches `a`	YES	3.6	no	n/a	no	YES	YES	YES	no	YES	YES	YES	YES	YES	YES	YES	n/a	ECMA	n/a	n/a	n/a	n/a	n/a	n/a
Exact spacing	`(?-x)` in the middle of the regex or a group `(?-x:regex)`	Turn off free-spacing to make whitespace significant after previously ignoring it.	`(?-x:a #b)` matches `a #b`	YES	3.6	no	n/a	no	YES	YES	YES	no	YES	7.0	YES	YES	YES	YES	YES	n/a	ECMA 1.42	n/a	n/a	n/a	n/a	n/a	n/a
Exact spacing	`(?t)` at the start of the regex	Turn on “tight” or exact spacing mode to treat whitespace and # characters as literals.	`(?t)a#b` matches `a#b`	no	no	n/a	n/a	no	no	no	no	no	no	no	no	no	no	no	no	n/a	no	YES	n/a	n/a	n/a	n/a	n/a
Free-spacing	`(?xx)`	Turn on free-spacing mode to ignore whitespace between regex tokens and allow # comments, both inside and outside character classes.	`(?xx)[ a]` matches `a` but not	no	no	error	n/a	no	no	no	no	default error	5.26 5.22 error	no	10.30	7.3.0	no	4.0.0	no	n/a	no	no	n/a	n/a	n/a	n/a	n/a
Case insensitive	`(?i)`	Turn on case insensitivity.	`(?i)a` matches `a` and `A`.	YES	YES	YES	n/a	YES	YES	YES	YES	default	YES	YES	YES	YES	YES	YES	YES	n/a	ECMA	YES	n/a	n/a	n/a	n/a	n/a
Case sensitive	`(?c)`	Turn on case sensitivity.	`(?c)a` matches `a` but not `A`.	no	no	no	n/a	no	no	no	no	no	no	no	no	no	no	no	no	n/a	no	YES	n/a	n/a	n/a	n/a	n/a
Single-line	`(?s)`	Make the dot match all characters including line break characters.	`(?s).*` matches `ab\n\ndef` in `ab\n\ndef`	YES	YES	YES	n/a	YES	YES	YES	YES	default	YES	YES	YES	YES	YES	YES	no	n/a	ECMA	no	n/a	n/a	n/a	n/a	n/a
Multi-line	`(?m)`	Make `^` and `$` match at the start and end of each line.	`(?m)^.` matches `a` and `d` in `ab\n\ndef`	YES	YES	YES	n/a	YES	YES	YES	YES	default	YES	YES	YES	YES	YES	YES	no	n/a	ECMA	no	n/a	n/a	n/a	n/a	n/a
Single-line	`(?m)`	Make the dot match all characters including line break characters.	`(?m).*` matches `ab\n\ndef` in `ab\n\ndef`	no	no	no	n/a	no	no	no	no	no	no	no	no	no	no	no	YES	n/a	no	no	n/a	n/a	n/a	n/a	n/a
Tcl single-line	`(?s)`	Make `^` and `$` match at the start and end of the string only. Make the dot match all characters including line break characters.	`(?s)^.{3}` matches only `ab\n` in `ab\n\ndef`	no	no	no	n/a	no	no	no	no	no	no	no	no	no	no	no	no	n/a	no	YES	n/a	n/a	n/a	n/a	n/a
Tcl multi-line	`(?m)`	Make `^` and `$` match at the start and end of each line. Do not allow the dot and negated character classes to match line break characters.	`(?m)^.` matches `a` and `d` in `ab\n\ndef`	no	no	no	n/a	no	no	no	no	no	no	no	no	no	no	no	no	n/a	no	YES	n/a	n/a	n/a	n/a	n/a
Tcl multi-line	`(?n)`	Make `^` and `$` match at the start and end of each line. Do not allow the dot and negated character classes to match line break characters.	`(?n)^.` matches `a` and `d` in `ab\n\ndef`	no	no	no	n/a	no	no	no	no	no	no	no	no	no	no	no	no	n/a	no	YES	n/a	n/a	n/a	n/a	n/a
Tcl “partial” newline-sensitive	`(?p)`	Make `^` and `$` match at the start and end of the string only. Do not allow the dot and negated character classes to match line break characters.	`(?p)^.*` matches only `ab` in `ab\n\ndef`	no	no	no	n/a	no	no	no	no	no	no	no	no	no	no	no	no	n/a	no	YES	n/a	n/a	n/a	n/a	n/a
Tcl “weird” newline-sensitive	`(?w)`	Make `^` and `$` match at the start and end of each line. Make the dot match all characters including line break characters.	`(?w)^.` matches `a`, the first `\n`, and `d` in `ab\n\ndef`	no	no	no	n/a	no	no	no	no	no	no	no	no	no	no	no	no	n/a	no	YES	n/a	n/a	n/a	n/a	n/a
Explicit capture	`(?n)`	Plain parentheses are non-capturing groups instead of numbered capturing groups. Only named capturing groups actually capture.	`(?n)(a\|b)c` is the same as `(?:a\|b)c`	YES	no	no	n/a	YES	YES	no	no	no	5.22	no	10.30	7.3.0	no	4.0.0	no	n/a	no	no	n/a	n/a	n/a	n/a	n/a
Duplicate named groups	`(?J)`	Allow multiple named capturing groups to share the same name.	`(?J)(?:(?'x'a)\|(?'x'b))\k'x'` matches `aa` or `bb`	no	no	no	n/a	no	no	no	no	no	no	6.7	YES	YES	YES	YES	no	n/a	no	no	n/a	n/a	n/a	n/a	n/a
Ungreedy quantifiers	`(?U)`	Switches the syntax for greedy and lazy quantifiers. Its use is strongly discouraged because it confuses the meaning of the standard quantifier syntax.	`(?U)a` is lazy and `(?U)a?` is greedy	no	no	no	n/a	no	no	no	no	default	no	YES	YES	YES	YES	YES	no	n/a	no	no	n/a	n/a	n/a	n/a	n/a
UNIX lines	`(?d)`	When anchors match at line breaks and when the dot does not match line breaks, make them recognize only the line feed character as a line break.	`(?dm)^.` matches `a` and `c` in `a\rb\nc`	no	no	no	n/a	no	no	YES	YES	no	no	no	no	no	no	no	no	n/a	no	no	n/a	n/a	n/a	n/a	n/a
UAX 29 word boundaries	`(?w)`	Word boundaries match according to Unicode Standard Annex 29 instead of matching between a word and non-word character.	`(?w)\b.\b` matches `!` because the WB1 and WB2 rules in UAX 29 treat the start and end of the string as a word boundary. `(?-w)\b` fails to match because the string `!` has no word characters.	no	no	no	n/a	no	no	no	YES	no	no	no	no	no	no	no	no	n/a	no	no	n/a	n/a	n/a	n/a	n/a
BRE	`(?b)`	Interpret the regular expression as a POSIX BRE.	`(?b)a\+` matches `aaa`	no	no	no	n/a	no	no	no	no	no	no	no	no	no	no	no	no	n/a	no	YES	n/a	n/a	n/a	n/a	n/a
ERE	`(?e)`	Interpret the regular expression as a POSIX ERE	`(?e)[a\]+` matches `a\a\a`	no	no	no	n/a	no	no	no	no	no	no	no	no	no	no	no	no	n/a	no	YES	n/a	n/a	n/a	n/a	n/a
Literal	`(?q)`	Interpret the regular expression as a literal string (excluding the modifier).	`(?q)[a\]+` matches `[a\]+` literally.	no	no	no	n/a	no	no	no	no	no	no	no	no	no	no	no	no	n/a	no	YES	n/a	n/a	n/a	n/a	n/a
Extra syntax	`(?X)`	Treat letters that are escaped with a backslash and that don’t form a regex token as an error instead of as a literal.	`(?X)\q` is an error while `(?-X)\q` matches `q`	no	no	no	n/a	no	no	no	no	no	no	YES	no	5.5.0–7.2.34	YES	2.14.0–3.6.3	no	n/a	no	no	n/a	n/a	n/a	n/a	n/a
ASCII classes	`(?a)`	Shorthand classes and/or POSIX classes match only ASCII characters.	`(?a)\d` matches only ASCII digits 0 to 9	no	3.0	no	n/a	no	no	no	no	no	YES	no	10.43	no	no	4.4.0	2.0	n/a	no	no	n/a	n/a	n/a	n/a	n/a
ASCII case folding	`(?a)`	Only ASCII characters are matched case insensitively when the regex is case insensitive.	`(?ai)aȗ` matches `Aȗ` but not `AȖ`	no	3.0	no	n/a	no	no	no	no	no	no	no	no	no	no	no	no	n/a	no	no	n/a	n/a	n/a	n/a	n/a
Unicode classes	`(?u)`	Shorthand classes and/or POSIX classes match Unicode characters.	`(?u)\d` matches `๑`	no	YES	no	n/a	no	no	no	no	no	YES	no	no	no	no	no	2.0	n/a	no	no	n/a	n/a	n/a	n/a	n/a
Unicode case folding	`(?u)`	All characters are matched case insensitively when the regex is case insensitive.	`(?ui)aȗ` matches `AȖ`	no	2.5	no	n/a	no	no	YES	no	no	no	no	no	no	no	no	no	n/a	no	no	n/a	n/a	n/a	n/a	n/a
Case insensitive	`(?u)`	Turn on case insensitivity.	`(?ui)aȗ` matches `AȖ`	no	no	no	n/a	no	no	5 only	no	no	no	no	no	no	no	no	no	n/a	no	no	n/a	n/a	n/a	n/a	n/a
Unicode classes and case folding	`(?U)`	Shorthand classes and/or POSIX classes match Unicode characters. Implies `(?u)` to turn on Unicode case folding.	`(?U)\dȗ` matches `๑Ȗ`	no	no	no	n/a	no	no	7	no	no	no	no	no	no	no	no	no	n/a	no	no	n/a	n/a	n/a	n/a	n/a
Separate ASCII/Unicode case folding	`(?aa)`	All characters are matched case insensitively when the regex is case insensitive, but ASCII characters cannot match non-ASCII characters or vice versa.	`(?aai)\x{212A}` matches the Kelvin symbol `K` but not the letter `k`	no	no	no	n/a	no	no	no	no	no	YES	no	no	no	no	no	no	n/a	no	no	n/a	n/a	n/a	n/a	n/a
Separate ASCII/Unicode case folding	`(?r)`	All characters are matched case insensitively when the regex is case insensitive, but ASCII characters cannot match non-ASCII characters or vice versa.	`(?aai)\x{212A}` matches the Kelvin symbol `K` but not the letter `k`	no	no	no	n/a	no	no	no	no	no	no	no	10.43	no	no	4.4.0	no	n/a	no	no	n/a	n/a	n/a	n/a	n/a
Default classes and case folding	`(?d)`	Shorthand and POSIX classes and case folding are reset to their defaults.		no	no	no	n/a	no	no	no	no	no	YES	no	no	no	no	no	2.0	n/a	no	no	n/a	n/a	n/a	n/a	n/a
Locale settings	`(?l)`	Shorthand and POSIX classes and case folding use the active locale.		no	no	no	n/a	no	no	no	no	no	YES	no	no	no	no	no	no	n/a	no	no	n/a	n/a	n/a	n/a	n/a
Locale settings	`(?L)`	Shorthand and POSIX classes and case folding use the active locale.		no	2.4–3.5	no	n/a	no	no	no	no	no	no	no	no	no	no	no	no	n/a	no	no	n/a	n/a	n/a	n/a	n/a
Locale settings		Mode modifiers that change shorthand and POSIX classes and/or case folding can be used in the middle of the regex.		no	no	no	n/a	no	no	YES	no	no	YES	no	10.43	no	no	4.4.0	2.0	n/a	no	n/a	n/a	n/a	n/a	n/a	n/a
Feature	Syntax	Description	Example	JGsoft	Python	JavaScript	VBScript	XRegExp	.NET	Java	ICU	RE2	Perl	PCRE	PCRE2	PHP	Delphi	R	Ruby	std::regex	Boost	Tcl	POSIX	GNU	Oracle	XML	XPath

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