Master JavaScript Regex: String Methods and RegExp Object Explained

Overall, JavaScript involves regular expressions in two areas: string handling and the RegExp object itself. Below we introduce the specific usage of each.

String methods

Four String methods support regular expressions:

search() – Takes a regular expression and returns the index of the first matching substring, or –1 if none. It converts a non‑regex argument to a RegExp and does not support global searches.

replace() – Performs search‑and‑replace. The first argument is a RegExp, the second is the replacement string or a function. When a function is used, its parameters are the matched substring, captured groups, the index, and the original string.

match() – Takes a RegExp and returns an array containing the match results. It supports global matching; with the g flag it returns all matches, otherwise it returns an array where index 0 is the full match and subsequent indices are captured groups, plus index and input properties.

split() – Splits a string by a RegExp. It accepts an optional second argument that limits the length of the resulting array.

RegExp object

Each RegExp instance has five properties:

source – Read‑only string containing the pattern text.

global – Read‑only boolean indicating whether the g flag is set.

ignoreCase – Read‑only boolean indicating whether the i flag is set.

multiline – Read‑only boolean indicating whether the m flag is set.

lastIndex – Read‑write integer that stores the position at which to start the next match.

The RegExp constructor RegExp(pattern, flags) accepts a pattern string and optional flags (g, i, m, etc.). Using the constructor allows dynamic creation of regular expressions.

Key RegExp methods:

exec() – Similar to String.match() but takes a string argument. Returns null if no match; otherwise returns an array with the same structure as a non‑global match(). It updates lastIndex to enable iterative matching.

test() – Returns true if the pattern matches the given string, otherwise false. It also updates lastIndex for global patterns.

compile() – Recompiles the RegExp with a new pattern or flags, improving performance when the same pattern is used repeatedly in loops.