JavaScript RegExp m Modifier

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.

let regex = /^hello/m;
let str = "world\nhello\nJavaScript";
let match = str.match(regex);
console.log(match); 

[
  '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

let regex = /^error/m;
let str = "info: everything is fine\nerror: something went wrong\ninfo: all good";
let matches = str.match(regex);
console.log(matches); 

[
  '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

let regex = /fine$/m;
let str = "info: everything is fine\nerror: something went wrong";
let match = str.match(regex);
console.log(match); 

[
  '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

let regex = /^\w+/gm;
let str = "line1\nline2\nline3";
let matches = str.match(regex);
console.log(matches); 

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

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

4. Validating Multiline Input

let regex = /^(error|info):/m;
let str = "error: this is an error\ninfo: this is information";
if (regex.test(str)) {
    console.log("Valid log format.");
} else {
    console.log("Invalid log format.");
}

Valid log format.

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

5. Removing Blank Lines

let regex = /^\s*$/gm;
let str = "line1\n\nline2\n\n";
let cleaned = str.replace(regex, "");
console.log(cleaned); 

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.

Recommended Links:

• JavaScript RegExp Complete Reference
• JavaScript Cheat Sheet-A Basic guide to JavaScript
• JavaScript Tutorial