Regular Expressions -Modifiers (Flags)

Regular expression modifiers (also called pattern modifiers or flags) are special directives used to change the matching behavior of a regular expression.

Flags are also called modifiers. Flags in regular expressions are used to specify additional matching strategies.

Flags are not written inside the regular expression; they are placed outside the expression, with the following format:

/pattern/flags

Common Modifiers

The following table lists commonly used modifiers in regular expressions:

1. i(ignore case) - Ignore case

  • Makes matching case-insensitive

  • Example:/abc/iCan match "abc", "Abc", "ABC", etc.

  • Supported languages: Almost all regular expression implementations (JavaScript, PHP, Python, etc.)

2. g(global) - Global match

  • Find all matches instead of stopping after the first match

  • Example: In the string "ababab",/ab/gMatches all three "ab"

  • Supported languages: JavaScript, PHP, etc.

3. m(multiline) - Multiline mode

  • Changes^and$the behavior so that it matches the beginning and end of each line, not just the beginning and end of the entire string.

  • Example: In a multiline string,/^abc/mMatches "abc" at the beginning of each line

  • Supported languages: JavaScript, PHP, Python, Perl, etc.

4. s(single line/dotall) - Single line mode

  • Makes the dot.match all characters, including newline characters

  • In JavaScript it is called "dotall" mode, using the/smodifier

  • Example:/a.b/sCan match "a\nb"

  • Supported languages: PHP, Perl, Python (asre.DOTALL)、JavaScript(ES2018+)

5. u(unicode) - Unicode mode

  • Enables full Unicode support

  • Correctly handles UTF-16 surrogate pairs and Unicode character properties

  • Example:/\p{Script=Greek}/uCan match Greek letters

  • Supported languages: JavaScript, PHP, etc.

6. y(sticky) - Sticky matching

  • Starts matching from the current position of the target string (using thelastIndexproperty)

  • Similar to^anchor, but it targets the starting position of the match

  • Example: In JavaScript,/a/ywill startlastIndexmatching "a" from

  • Supported languages: JavaScript

7. x(extended) - Extended mode

  • Ignores whitespace and comments in the pattern, making the regular expression more readable

  • Example: In PHP,/a b c/xis equivalent to/abc/

  • Supported languages: PHP, Perl, Python (asre.VERBOSE)

g modifier

The g modifier finds all matches in a string:

Example

Find "example" in the string:

var str="Google example taobao example"; var n1=str.match(/example/); //Find the first match var n2=str.match(/example/g); //Find all matches

Try it Yourself »

i modifier

The i modifier performs case-insensitive matching, example as follows:

Example

Find "example" in the string:

var str="Google example taobao example"; var n1=str.match(/example/g); //Case-sensitive var n2=str.match(/example/gi); //Case-insensitive

Try it Yourself »

m modifier

The m modifier makes^and$match the beginning and end positions of each line in a piece of text.

With only g, it matches only the first line; after adding m, multiline matching is achieved.

In the example below, the string uses\nto break lines:

Example

Find "example" in the string:

var str="examplegoogle\ntaobao\nexampleweibo"; var n1=str.match(/^example/g); //Match one var n2=str.match(/^example/gm); //Multiline match

Try it Yourself »

s modifier

By default, the dot.matches any character except newline\nAfter adding s,.includes newline characters\n。

Example of the s modifier:

Example

Find in the string:

var str="google\nexample\ntaobao"; var n1=str.match(/google./); //Without using s, \n cannot be matched var n2=str.match(/example./s); //Using s, \n is matched

Try it Yourself »

Extended Notes

Supplementary table of language-specific modifiers:

Language Specific modifier Description
PHP A Anchors the pattern to the beginning of the string
D $Matches only the end of the string (excluding trailing newline)
U Reverses the greediness of quantifiers (makes all quantifiers non-greedy)
Python re.A make\w,\W,\b,\Betc. match only ASCII characters
re.L Determined according to locale settings\w,\Wmeaning of etc.
JS(ES2022) d Generates an indices property for match results (containing start and end indices of match positions)

Example table of modifier combinations:

Combination Effect
gi Global match + ignore case (e.g., find all forms of the word "email")
ims Ignore case + multiline mode + dot matches newline (commonly used in log analysis)
gu Global match + Unicode support (e.g., find all Unicode emojis)

Inline modifier table (PCRE/Perl style):

Syntax Scope Example
(?i) Enable ignore case a(?i)bc→ Matches "aBc", "aBC"
(?-i) Disable ignore case a(?i)b(?-i)c→ Matches only "aBc"
(?i:...) Only effective within parentheses a(?i:b)c→ Matches "aBc", "abc"

Note: Implementations of modifiers may vary across languages. It is recommended to consult the documentation of the specific language when using them.

Other extensions