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JavaScript RegExp m Modifier

Last Updated : 5 Aug, 2025

The m modifier in JavaScript regular expressions stands for "multiline". It alters the behavior of the ^ (caret) and $ (dollar) anchors, allowing them to match the start or end of any line in a multiline string, rather than just the start or end of the entire string.


Output
[
 'hello',
 index: 6,
 input: 'world\nhello\nJavaScript',
 groups: undefined
]

The ^hello pattern matches "hello" at the beginning of the second line because the m modifier treats each line separately.

Syntax:

let regex = /pattern/m;
  • pattern: The regular expression to apply.
  • m: Enables multiline behavior for ^ and $.

Key Points

  • Without the m modifier, ^ and $ match only the start and end of the entire string.
  • With the m modifier, ^ and $ match the start and end of each line in a multiline string.
  • Useful when processing text with multiple lines, such as logs or formatted input.

Real-World Examples

1. Matching the Start of Any Line


Output
[
 'error',
 index: 25,
 input: 'info: everything is fine\nerror: something went wrong\ninfo: all good',
 groups: undefined
]

The ^error pattern matches "error" at the beginning of the second line.

2. Matching the End of Any Line


Output
[
 'fine',
 index: 20,
 input: 'info: everything is fine\nerror: something went wrong',
 groups: undefined
]

The $fine pattern matches "fine" at the end of the first line.

3. Finding All Matches Across Lines


Output
[ 'line1', 'line2', 'line3' ]

The ^\w+ pattern matches the first word in each line because of the m modifier.

4. Validating Multiline Input


Output
Valid log format.

The ^(error|info): pattern ensures each line begins with "error:" or "info:".

5. Removing Blank Lines


Output
line1

line2

The pattern ^\s*$ matches empty lines in the string, and the m modifier ensures all lines are checked.

Common Patterns Using m

  • Match All Lines Starting with a Word:
/^word/m
  • Match Empty Lines:
/^\s*$/m
  • Extract Lines Ending with a Period:
/\.$/m
  • Validate Log Formats:
/^(info|error|debug):/m
  • Split Text at Line Boundaries:
str.split(/\r?\n/);

Why Use the m Modifier?

  • Process Multiline Input: Essential for handling multi-line text files or logs.
  • Enhanced Flexibility: Enables patterns to match within individual lines, not just the entire string.
  • Text Parsing: Simplifies tasks like validation, filtering, and transformation of multi-line text.

Conclusion

The m modifier is an invaluable tool when working with multi-line strings in JavaScript. It ensures efficient line-by-line pattern matching and processing.

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