Regular Expressions – Flags (Modifiers) |

Regular Expressions - Flags (Modifiers)

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

Flags are also known as modifiers. The flags of a regular expression are used to specify additional matching strategies.

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

/pattern/flags

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Common Flags

The following table lists common flags used with regular expressions:

1. i (ignore case) - Case-Insensitive Matching

• Makes the match case-insensitive.
• Example: /abc/i can match "abc", "Abc", "ABC", etc.
• Supported in: Almost all regular expression implementations (JavaScript, PHP, Python, etc.)

2. g (global) - Global Matching

• Finds all matches, rather than stopping after the first match.
• Example: In the string "ababab", /ab/g will match all three "ab"s.
• Supported in: JavaScript, PHP, etc.

3. m (multiline) - Multiline Mode

• Changes the behavior of ^ and $ to match the start and end of each line, not just the start and end of the entire string.
• Example: In a multiline string, /^abc/m will match "abc" at the start of each line.
• Supported in: JavaScript, PHP, Python, Perl, etc.

4. s (single line/dotall) - Single Line Mode

• Makes the dot . match all characters, including newline characters.
• In JavaScript, this is called "dotall" mode, using the /s flag.
• Example: /a.b/s can match "anb".
• Supported in: PHP, Perl, Python (as re.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}/u can match Greek letters.
• Supported in: JavaScript, PHP, etc.

6. y (sticky) - Sticky Matching

• Starts matching from the current position in the target string (using the lastIndex property).
• Similar to the ^ anchor, but for the starting position of the match.
• Example: In JavaScript, /a/y will match "a" starting from lastIndex.
• Supported in: 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/x is equivalent to /abc/.
• Supported in: PHP, Perl, Python (as re.VERBOSE).

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The g Flag

The g flag can find all matches in a string:

Example

Find "" in a string:

var str="Google  taobao ";
var n1=str.match(//);
var n2=str.match(//g);

Try it »

The i Flag

The i flag performs a case-insensitive match. Example:

Example

Find "" in a string:

var str="Google  taobao ";
var n1=str.match(//g);
var n2=str.match(//gi);

Try it »

The m Flag

The m flag allows ^ and $ to match the start and end positions of each line in a block of text.

g only matches the first line; adding m enables multiline matching.

The following example uses n for line breaks in the string:

Example

Find "" in a string:

var str="tutorialgooglen taobaon tutorialweibo";
var n1=str.match(/^/g);
var n2=str.match(/^/gm);

Try it »

The s Flag

By default, the dot . matches any character except the newline character n. With the s flag, . includes the newline character n.

Example of the s flag:

Example

Find in a string:

var str="googlen tutorialn taobao";
var n1=str.match(/google./);
var n2=str.match(/./s);

Try it »

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Extended Explanation

Language-Specific Flags Supplement Table:

Language	Specific Flag	Description
PHP	A	Anchors the pattern to the start of the string.
	D	$ matches only the end of the string (not the end of a line with a trailing newline).
	U	Inverts the greediness of quantifiers (makes all quantifiers non-greedy).
Python	re.A	Makes w, W, b, B, etc., match only ASCII characters.
	re.L	Determines the meaning of w, W, etc., based on locale settings.
JS (ES2022)	d	Generates an indices property for the match result (containing start and end indices of the match).

Flag Combination Examples Table:

Combination	Effect
gi	Global matching + case-insensitive (e.g., finding all forms of the word "email").
ims	Case-insensitive + multiline mode + dot matches newline (often used in log analysis).
gu	Global matching + Unicode support (e.g., finding all Unicode emojis).

Inline Flags Table (PCRE/Perl Style):

Syntax	Scope	Example
(?i)	Enables case-insensitive matching.	a(?i)bc → matches "aBc", "aBC"
(?-i)	Disables case-insensitive matching.	a(?i)b(?-i)c → only matches "aBc"
(?i:...)	Applies only within the parentheses.	a(?i:b)c → matches "aBc", "abc"

Note: Implementations of flags may vary across different languages. It is recommended to refer to the specific language's documentation when using them.