JavaScript RegExp g Modifier
The g (global) modifier in JavaScript regular expressions is used to perform a global search. It ensures the pattern is matched multiple times throughout the entire string, rather than stopping at the first match.
let regex = /cat/g;
let str = "cat, caterpillar, catch a cat";
let matches = str.match(regex);
console.log(matches);
Output
[ 'cat', 'cat', 'cat', 'cat' ]
The g modifier finds all occurrences of “cat” in the string, even if they appear multiple times.
Syntax:
/pattern/gKey Points
- Global Search: Finds all matches in the string, not just the first.
- Combination: Can be combined with other flags like i (case-insensitive) and m (multiline).
- Match Iteration: Useful for iterating over all matches using methods like match(), exec(), or loops.
Real-World Examples
1. Finding All Matches
let regex = /dog/g;
let str = "dog, doggy, dogs are friends";
let matches = str.match(regex);
console.log(matches);
Output
[ 'dog', 'dog', 'dog' ]
Here, the g modifier ensures all matches of “dog” are found in the string.
2. Counting Word Occurrences
let str = "apple orange apple banana apple";
let regex = /apple/g;
let count = (str.match(regex) || []).length;
console.log(count);
Output
3
Using the g modifier with match(), we can count how many times “apple” appears.
3. Replacing All Matches
let str = "foo bar foo baz foo";
let regex = /foo/g;
let result = str.replace(regex, "qux");
console.log(result);
Output
qux bar qux baz qux
The g modifier ensures all occurrences of “foo” are replaced with “qux”.
4. Iterating Over Matches
let regex = /\d+/g;
let str = "The price is 20 dollars and 30 cents.";
let match;
while ((match = regex.exec(str)) !== null) {
console.log(`Found: ${match[0]} at index ${match.index}`);
}
Output
Found: 20 at index 13 Found: 30 at index 28
The g modifier with exec() allows iterating over all numeric matches in the string.
5. Case-Insensitive Global Search
let regex = /hello/gi;
let str = "Hello, HELLO, hello";
let matches = str.match(regex);
console.log(matches);
Output
[ 'Hello', 'HELLO', 'hello' ]
Combining g with i makes the search case-insensitive while finding all matches.
When Not to Use the g Modifier
If you only need the first match, do not use g. For example, str.match() without g returns only the first match as an array.
let str = "repeat repeat repeat";
console.log(str.match(/repeat/));
Output
[ 'repeat', index: 0, input: 'repeat repeat repeat', groups: undefined ]
When using test() in loops, the g modifier can cause unexpected results due to its effect on the lastIndex property.
Why Use the g Modifier?
- Comprehensive Matching: Ensures no match is missed in the target string.
- Text Analysis: Useful for tasks like word frequency analysis or log parsing.
- String Manipulation: Ideal for global replacements and transformations.
Conclusion
The g modifier is essential for working with patterns that occur multiple times in a string, making it a powerful tool for developers in text processing and manipulation.
Recommended Links:
- JavaScript RegExp Complete Reference
- JavaScript Cheat Sheet-A Basic guide to JavaScript
- JavaScript Tutorial
