Regex for Whitespace

Whitespace in Regular Expressions

Whitespace handling is one of the most common tasks in string processing — trimming user input, collapsing sloppy formatting, splitting on any whitespace, or detecting where text begins on a line. Regex has purpose-built shortcuts for whitespace that work identically in every major language: \s matches any whitespace character, \S matches any non-whitespace. Master these two and 90% of your whitespace problems disappear.

The Whitespace Character Class

\s matches any of the following characters:

•   — Space (U+0020)
• \t — Tab (U+0009)
• \n — Newline / line feed (U+000A)
• \r — Carriage return (U+000D)
• \f — Form feed (U+000C)
• \v — Vertical tab (U+000B)

In Unicode-aware modes (JavaScript with the u flag, Python 3 by default, .NET), \s also matches non-breaking space (U+00A0), en space, em space, and all other Unicode whitespace characters. This is usually what you want when handling real-world text pasted from Word documents or web pages.

The Core Patterns

Match one or more whitespace characters

/\s+/g          // any run of whitespace — the workhorse
/\s/g           // exactly one whitespace character
/\s{2,}/g       // two or more (find double spaces)
/\s*/g          // zero or more (rarely useful — matches empty positions)

Match only spaces (not tabs/newlines)

/ +/g           // literal space, one or more
/[ ]+/g         // same thing, more explicit
/[ \t]+/g      // spaces or tabs (horizontal whitespace only)

Trim leading and trailing whitespace

/^\s+|\s+$/g   // classic trim pattern
/^\s+/         // leading only
/\s+$/         // trailing only

Non-whitespace

/\S+/g          // any run of non-whitespace — useful for word extraction
/\S/g           // any single non-whitespace character

Language-Specific Usage

JavaScript

// Remove ALL whitespace
'  hello   world  '.replace(/\s+/g, '');
// → 'helloworld'

// Collapse whitespace + trim
'  hello   world  '.replace(/\s+/g, ' ').trim();
// → 'hello world'

// Trim only
'  hello  '.replace(/^\s+|\s+$/g, '');
// → 'hello'  (or just use .trim())

// Split on any whitespace run
'the  quick\tbrown\nfox'.split(/\s+/);
// → ['the', 'quick', 'brown', 'fox']

// Count words
str.trim().split(/\s+/).length;

Python

import re

# Remove all whitespace
re.sub(r'\s+', '', '  hello   world  ')
# → 'helloworld'

# Collapse and trim
' '.join('  hello   world  '.split())    # Pythonic, no regex needed
re.sub(r'\s+', ' ', s).strip()          # regex version

# Split on any whitespace
s.split()                                # default splits on any whitespace run
re.split(r'\s+', s.strip())            # explicit regex version

# Count words
len(re.findall(r'\S+', s))

PHP

// Remove all whitespace
preg_replace('/\s+/', '', $s);

// Collapse and trim
trim(preg_replace('/\s+/', ' ', $s));

// Split on any whitespace
preg_split('/\s+/', trim($s));

// PHP built-in: trim($s) is simpler for basic trimming

Common Pitfalls

The dot-matches-newline problem

In most regex flavours, . matches any character except newlines. If your text has embedded newlines and you want to match everything, use [\s\S] (the "whitespace or non-whitespace" trick — literally any character) or enable the DOTALL / single-line flag: /pattern/s in Perl/PHP/Python, /pattern/s also in JavaScript (ES2018+).

Non-breaking space traps

Text copied from Word or webpages often contains U+00A0 (non-breaking space) which LOOKS identical to a space but is not U+0020. In JavaScript (no u flag), \s DOES match it. In some other flavours it does not. If you have to trim non-breaking spaces explicitly: /[\s\u00a0]+/g.

Zero-width matches

/\s*/ (star, not plus) matches ZERO or more whitespace characters — which matches the empty position at every character boundary. Using replace(/\s*/g, 'X') on "abc" produces "XaXbXcX". Almost always use + (one or more) instead of *.

Multiline mode changes ^ and $

By default, ^ and $ match the start and end of the whole string. With the m flag, they also match the start and end of each line — so /^\s+/gm strips leading whitespace from every line, not just the first.

Whitespace Cheatsheet

Testing Your Whitespace Regex

Use the live Regex Tester with this test string (paste it exactly, with the mixed whitespace intact):

   Leading spaces
mid   line   with   spaces
tab	here	and	there
trailing spaces   
     

• Try /\s+/g — should highlight every whitespace run
• Try / +/g — should skip tabs and newlines
• Try /\S+/g — should highlight the words only
• Try /^\s+/gm — should highlight the indentation on each line

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