Regular Expressions
Regular expressions are patterns used to match character combinations in strings.
Featured Developer Sponsor • Zero-Token Protection
📋 Copy JavaScript Regex Named Capture Groups Snippet
// Named Capture Groups & Testing
const logPattern = /\[(?<level>INFO|WARN|ERROR)\]\s+(?<msg>.*)/u;
const match = logPattern.exec("[ERROR] Connection refused to database");
if (match) {
const { level, msg } = match.groups;
console.log(`Level: ${level} | Message: ${msg}`);
}⚠️ 5 Fatal Traps & JavaScript Pitfalls
Trap #1: Global Flag (/g) Stateful lastIndex Bug
When using the
/g flag, RegExp objects store internal state on lastIndex. Calling regex.test(str) repeatedly alternates between returning true and false! Reset regex.lastIndex = 0.Trap #2: Catastrophic Backtracking (ReDoS Vulnerability)
Nested quantifiers like
(a+)+$ cause exponential backtracking on non-matching inputs, freezing the CPU for minutes (Regular Expression Denial of Service).Trap #3: Unescaped User Input in Dynamic new RegExp()
Passing unsanitized strings to
new RegExp(input) crashes if the user types regex special characters like [ or ?. Escape special characters first.Trap #4: Greedy vs. Lazy Quantifier Confusion
By default, quantifiers like
.* are greedy and match as much text as possible. Append ? (e.g. .*?) for lazy minimal matching.Trap #5: Omitting the Unicode (/u) Flag
Without the
u flag, regular expressions treat emojis as surrogate pairs of separate characters, breaking matching on non-ASCII symbols.💬 Frequently Asked Questions
Sponsored Utility
While You're Here
Sponsored Recommendations
Advertisement