JavaScript RegExp Sticky Property

The sticky property in JavaScript regular expressions determines whether the y (sticky) flag is enabled. A sticky regular expression searches for a match starting from the current position in the target string and does not search further if a match isn't found at that exact point.

// Regular expression without 'y' flag
let regex1 = /test/;
console.log(regex1.sticky);

// Regular expression with 'y' flag
let regex2 = /test/y;
console.log(regex2.sticky);

false
true

• regex1 is not sticky because it doesn't have the y flag.
• regex2 is sticky, so it only attempts to match the test pattern at the exact position specified by the lastIndex property of the regex.

Syntax

regex.sticky

Key Points

• Read-Only: The property is read-only and reflects the presence of the y flag.
• Precise Position Matching: A sticky regex remain strictly to the lastIndex property during matching.
• Use Case: Often used in conjunction with the exec() method to parse strings iteratively without skipping over parts.

Real-World Examples of the sticky Property

1. Parsing Consecutive Words

let s = "hello hello";
let regex = /hello/y;

regex.lastIndex = 0;
console.log(regex.exec(s)); 

regex.lastIndex = 5;
console.log(regex.exec(s));

regex.lastIndex = 6;
console.log(regex.exec(s));

[ 'hello', index: 0, input: 'hello hello', groups: undefined ]
null
[ 'hello', index: 6, input: 'hello hello', groups: undefined ]

The y flag ensures the regex matches only when the lastIndex points to the correct position in the string.

2. Matching Numbers in a CSV

let csv = "123,456,789";
let regex = /\d+/y;

regex.lastIndex = 0;
while ((match = regex.exec(csv)) !== null) {
    console.log(`Matched: ${match[0]} at index ${regex.lastIndex}`);
    regex.lastIndex++;
}

Matched: 123 at index 3
Matched: 456 at index 7
Matched: 789 at index 11

3. Strict Match at Current Position

let s = "sticky test";
let regex = /sticky/y;

regex.lastIndex = 0;
console.log(regex.exec(s));

regex.lastIndex = 1;
console.log(regex.exec(s));

[ 'sticky', index: 0, input: 'sticky test', groups: undefined ]
null

4. Splitting a String with Sticky Matching

let s = "apple|banana|cherry";
let regex = /\w+/y;

let a = [];
regex.lastIndex = 0;

while ((match = regex.exec(s)) !== null) {
    a.push(match[0]);
    regex.lastIndex++;
}

console.log(a);

[ 'apple', 'banana', 'cherry' ]

5. Iterative Parsing Without Backtracking

Sticky regex is ideal for scenarios where parsing must progress linearly without revisiting skipped positions:

let s = "key=value;key2=value2";
let regex = /\w+=\w+/y;

regex.lastIndex = 0;
while ((match = regex.exec(s)) !== null) {
    console.log(`Match: ${match[0]} at position ${regex.lastIndex}`);
    regex.lastIndex++;
}

Match: key=value at position 9
Match: key2=value2 at position 21

Why Use the sticky Property?

• Efficient Parsing: Avoids unnecessary backtracking and skips.
• Precise Control: Matches only at the exact position specified by lastIndex.
• Stream Processing: Useful for sequential parsing of structured data, like CSVs or logs.