Regex Find and Replace Online — Test Before You Run It on Your Data

Running a regex find-and-replace on real data without testing first is how you corrupt a database, break a config file, or mess up a thousand file names. The browser tester lets you prove the replacement is correct before touching actual data.

Our regex tester shows you which text the pattern matches — the first step in any find-and-replace workflow. Once you confirm the matches are correct, you know your replacement will apply to exactly the right text. This guide walks through the find-and-replace workflow in the most common environments.

The Two-Step Workflow: Find First, Replace Second

Good regex find-and-replace practice is always two steps:

1. Test the find pattern — paste your test input into the tester, verify every highlighted match is something you want to replace, and nothing is highlighted that you want to keep.
2. Run the replacement — only after the find is verified. Use the appropriate tool for your environment (see sections below).

This two-step approach prevents the most common find-and-replace disasters: replacements that change more than expected, replacements that miss edge cases, and replacements that apply to the wrong context.

Paste a representative sample of your real data into the tester — not a made-up example. Real data has edge cases that constructed examples miss every time.

Using Capture Groups in Replacements

The real power of regex find-and-replace is referencing what you captured. A capture group (pattern) stores the matched text, and you reference it in the replacement string using $1, $2, etc. (JavaScript/most tools) or \1, \2 (Python, sed).

Example — reformat a date from MM/DD/YYYY to YYYY-MM-DD:

• Find: (\d{2})\/(\d{2})\/(\d{4})
• Replace: $3-$1-$2 (JavaScript) or \3-\1-\2 (Python/sed)
• Input: 04/08/2026
• Output: 2026-04-08

Example — add quotes around CSV values that are missing them:

• Find: (?<!["\n])(\w[^,\n]*)
• Replace: "$1"

Example — strip HTML tags:

• Find: <[^>]+>
• Replace: (empty string)

Test each of these find patterns in the tester before running the replacement — confirm the match highlights exactly the text you want changed.

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Find and Replace in VS Code, Python, JavaScript, and sed

VS Code: Open Find (Ctrl+H), enable the regex icon (.*), enter your pattern in Find and your replacement in Replace. Use $1 for group references. Test your pattern here by seeing what highlights before clicking Replace All.

Python:

import re
result = re.sub(r'(\d{2})/(\d{2})/(\d{4})', r'\3-\1-\2', text)

Use raw strings (r'...') to avoid double-escaping. Use \1-style group references in the replacement.

JavaScript:

const result = str.replace(/(\d{2})\/(\d{2})\/(\d{4})/g, '$3-$1-$2');

Use $1-style group references. The g flag replaces all occurrences — without it, only the first match is replaced.

sed (Linux):

sed -E 's/(\d{2})\/(\d{2})\/(\d{4})/\3-\1-\2/g' input.txt

Use -E for ERE (no backslash-escaping for groups). Use \1-style references. Test the ERE pattern in the browser tester first.

Patterns That Cause Disasters Without Testing First

These find-and-replace operations regularly cause unintended data corruption when run without testing:

Greedy matching that consumes too much:
Find: <div.*> (greedy)
This matches from the first <div to the last > on the line — potentially a huge chunk including other tags. Use <div.*?> (non-greedy) or <div[^>]*> to match only the opening tag.

Replacing SQL values without escaping:
Pattern: 'O'Brien' matched as-is. Single quotes in replacement strings can break SQL. Test in staging, never production, for SQL replacements.

Multiline replacements in single-line mode:
If your pattern should match across line breaks, you need the s (dotAll) flag so . matches newlines. Without it, your pattern stops at each line end — usually with confusing results. Enable the s flag when testing patterns meant to span lines.

Global flag missing in JavaScript:
Without the g flag, str.replace(regex, replacement) only replaces the FIRST match. Add g to replace all occurrences. Verify in the tester how many matches your pattern has before running the replacement.

Frequently Asked Questions

How do I reference a capture group in my replacement string?

In JavaScript and most tools, use $1, $2, $3 to reference the first, second, third capture group. In Python, sed, and awk, use \1, \2, \3 instead. Named groups can be referenced by name: $ in JavaScript, \g in Python.

Why does my regex replace only the first match in JavaScript?

The g (global) flag is required to replace all occurrences. Without it, str.replace() only replaces the first match. Add g to your regex flags: /pattern/g. The tester shows all matches by default — this is equivalent to having the g flag on.

How do I replace text between two strings (start and end markers)?

Use a pattern like START.*?END with the s flag (to match across newlines if needed) and non-greedy matching (*?) to stop at the first END occurrence rather than the last. Test the match in the tester before running any replacement.