Common regex mistakes in JavaScript.

Some characters have special meaning in regular expressions. For example:

. * + ? ^ $ { } ( ) | [ ] \

If you want to match a literal dot instead of “any character”, you must escape it:

// wrong
/./

// correct
/\./

Without the g flag, JavaScript usually returns only the first match.

const text = "React Redux React";

// only first match
text.match(/React/);

// all matches
text.match(/React/g);

This is one of the most common regex mistakes when working with lists, logs, or repeated text.

Quantifiers like * and + are greedy by default. They try to match as much text as possible.

const text = "<div>Hello</div><div>World</div>";

// greedy
/<div>.*<\/div>/

// lazy
/<div>.*?<\/div>/

Lazy matching uses ? to stop matching earlier.

Broad patterns can accidentally match data you never intended to capture.

// too broad
/.+@.+/

// safer
/[a-zA-Z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,}/

Specific patterns are usually safer and easier to debug.

The m flag changes how ^ and $ behave.

const text = "first line
second line";

// without m
/^second/

// with m
/^second/m

Multiline mode allows regex anchors to work per line instead of only at the beginning or end of the entire string.

Complex regular expressions are difficult to maintain if everything is packed into one line.

Sometimes normal JavaScript string methods are easier and safer than a complicated regex.