Regex Tester

How to Use the Regex Tester

This free online regex tester lets you write and test regular expressions in real time with instant match highlighting. Enter your pattern in the regex field, set your desired flags, and type or paste your test string. Matches are highlighted immediately as you type, and a detailed match table shows each match along with its position and captured groups. All processing runs entirely in your browser using the native JavaScript RegExp engine.

Understanding Regex Flags

JavaScript supports several regex flags that modify matching behavior. g (global) finds all matches rather than stopping at the first one. i (case-insensitive) ignores case when matching letters. m (multiline) makes ^ and $ match the start and end of each line rather than the entire string. s (dotAll) makes the dot character match newline characters as well. u(unicode) enables full Unicode matching. You can combine multiple flags, for example "gi" for global case-insensitive matching.

Common Regex Patterns

Some commonly used patterns include \d+ for matching numbers, [a-zA-Z]+ for matching words, \b\w+@\w+\.\w+\b for basic email matching, and https?:\/\/\S+ for matching URLs. Captured groups using parentheses are shown in the match details table, making it easy to extract specific parts of your matches.

Tips for Writing Regex

Start simple and build complexity gradually. Use the real-time highlighting to verify each part of your pattern works before adding more. Remember to escape special characters like dots, brackets, and backslashes with a backslash when you want to match them literally. Captured groups with parentheses allow you to extract substrings from matches, which is useful when parsing structured text like log files or CSV data.

正規表現テスターの使い方

パターン欄に正規表現を入力し、テスト文字列を貼り付けるとリアルタイムでマッチ箇所がハイライト表示されます。 フラグ(g, i, mなど)を組み合わせて検索動作を変更でき、マッチ詳細テーブルで各マッチの位置やキャプチャグループも確認できます。 全ての処理はブラウザ内で完結し、データが外部に送信されることはありません。

すぐ使えるパターン

• 日本の郵便番号: /^\d{3}-?\d{4}$/
• メールアドレス（現実解）: /^[^\s@]+@[^\s@]+\.[^\s@]+$/
• URL抽出: /https?:\/\/\S+/g
• 全角英数を半角に寄せる前処理には String.prototype.normalize("NFKC") を併用

さらに多くの実例・ハマりどころは正規表現の実例集に まとめています。郵便番号・電話番号・和暦・クレジットカード番号など、日本でよく使うパターンを コピペできる形で掲載しています。

よくあるエラーと対処 / Common Errors and Fixes

言語別コード例 / Code Examples by Language

JavaScript

// Global match
const hits = text.match(/\d+/g) ?? [];

// Named capture
const m = "2026-04-19".match(/(?<y>\d{4})-(?<m>\d{2})-(?<d>\d{2})/);
console.log(m?.groups); // { y: "2026", m: "04", d: "19" }

// Replace
const slug = title.toLowerCase().replace(/[^a-z0-9]+/g, "-");

Python

import re
m = re.search(r"(?P<y>\d{4})-(?P<m>\d{2})-(?P<d>\d{2})", s)
if m: print(m.group("y"), m.group("m"), m.group("d"))

# All matches
nums = re.findall(r"\d+", s)

Go

import "regexp"
re := regexp.MustCompile(`\d+`)
for _, n := range re.FindAllString(s, -1) { /* ... */ }

Shell (grep -E)

grep -Eo '[0-9]{3}-[0-9]{4}' file.txt

Before / After 実例 / Before & After

例1: 郵便番号の抽出 / Extract Japanese postal codes

Input : "〒100-0001 東京都千代田区、〒150-0002"
Regex : /\d{3}-\d{4}/g
Match : ["100-0001", "150-0002"]

例2: URL 抽出 / Extract URLs

Input : "参考: https://devtoolspot.com と http://example.com"
Regex : /https?:\/\/\S+/g
Match : ["https://devtoolspot.com", "http://example.com"]

例3: キャッシュバスター削除 / Strip cache-buster query

Input : "style.css?v=1234567"
Regex : /\?v=\d+$/
Replace with "" → "style.css"

Why browser-only? / なぜブラウザ完結か

Regex work usually means testing patterns against real log lines, user input, or production strings — which often carry PII, session IDs, or internal URLs. DevToolBox runs RegExplocally using your browser's JavaScript engine. No sample strings travel over the network.

正規表現の検証は、生のログ・ユーザー入力・本番文字列を対象にすることが多く、 PII、セッション ID、内部 URL が混ざります。DevToolBox はブラウザの JavaScript エンジンでRegExp を実行します。サンプル文字列はネットワークに出ません。