Regular expressions (regex) are one of the most powerful tools in a developer's arsenal—and one of the most feared. This tutorial breaks down regex into digestible pieces, building from basic concepts to patterns you can use in real projects.
🎯 By the end of this tutorial, you'll be able to: Write patterns for email validation, password requirements, phone numbers, and more. You'll understand quantifiers, groups, lookaheads, and know when to use (or avoid) regex.
正規表現とは何ですか?
Regular expressions are patterns that describe sets of strings. They're used to:
- Search: Find text matching a pattern
- Validate: Check if input matches expected format
- Extract: Pull specific parts from text
- Replace: Transform text based on patterns
基本: リテラル文字
The simplest regex is just literal text:
Matches: "hello" in "hello world"
Does not match: "Hello" (case-sensitive by default)
文字クラス
Match any single character from a set:
| Pattern | Matches | Example |
|---|---|---|
[abc] |
a, b, or c | "cat" matches c |
[a-z] |
Any lowercase letter | "Hello" matches e, l, l, o |
[A-Z] |
Any uppercase letter | "Hello" matches H |
[0-9] |
Any digit | "abc123" matches 1, 2, 3 |
[^abc] |
NOT a, b, or c | "dog" matches d, o, g |
速記文字クラス
| Shorthand | Equivalent | Description |
|---|---|---|
\d |
[0-9] |
Any digit |
\D |
[^0-9] |
Not a digit |
\w |
[a-zA-Z0-9_] |
Word character |
\W |
[^a-zA-Z0-9_] |
Not a word character |
\s |
[ \t\n\r] |
Whitespace |
\S |
[^ \t\n\r] |
Not whitespace |
. |
(almost anything) | Any character except newline |
数量詞: いくつ?
| Quantifier | Meaning | Example |
|---|---|---|
* |
0 or more | ab*c matches "ac", "abc", "abbc" |
+ |
1 or more | ab+c matches "abc", "abbc", not "ac" |
? |
0 or 1 | colou?r matches "color" and "colour" |
{n} |
Exactly n | \d{4} matches "2026" |
{n,} |
n or more | \d{2,} matches "12", "123", "1234" |
{n,m} |
Between n and m | \d{2,4} matches "12", "123", "1234" |
✅ 貪欲 vs 怠け者
Quantifiers are "greedy" by default—they match as much as possible. Add ? to make them
"lazy" (match as little as possible). .* vs .*?
アンカー: 一致する場所
| Anchor | Position | Example |
|---|---|---|
^ |
Start of string/line | ^Hello matches "Hello world", not "Say Hello" |
$ |
End of string/line | world$ matches "Hello world", not "world peace" |
\b |
Word boundary | \bcat\b matches "cat" but not "category" |
グループとキャプチャ
Parentheses create groups for:
- Applying quantifiers to multiple characters:
(ab)+ - Capturing matched text for later use
- Creating alternatives:
(cat|dog)
例: グループのキャプチャ
Input: "555-123-4567"
Group 0 (full match): "555-123-4567"
Group 1: "555"
Group 2: "123"
Group 3: "4567"
非キャプチャグループ
Use (?:...) when you need grouping but don't need to capture:
実践的なパターン
電子メール検証 (基本)
Matches: [email protected], [email protected]
電話番号 (米国)
Matches: (555) 123-4567, 555-123-4567, 555.123.4567
パスワード (8 文字以上、大文字、小文字、数字)
Uses lookaheads to require different character types
URL
Matches: http://example.com, https://sub.domain.com/path
先読みと後読み
Match based on what comes before or after, without including it in the match:
| Type | Syntax | Description |
|---|---|---|
| Positive Lookahead | (?=...) |
Followed by ... |
| Negative Lookahead | (?!...) |
NOT followed by ... |
| Positive Lookbehind | (?<=...) |
Preceded by ... |
| Negative Lookbehind | (? |
NOT preceded by ... |
foo(?=bar) // matches "foo" in "foobar", not in "foobaz"
// Match $ amount (digit preceded by $)
(?<=\$)\d+ // matches "100" in "$100"
フラグ/修飾子
| Flag | Description |
|---|---|
i |
Case-insensitive matching |
g |
Global - find all matches, not just first |
m |
Multiline - ^ and $ match line boundaries |
s |
Dotall - . matches newlines too |
よくある間違い
- Forgetting to escape:
.,*,+,?, etc. have special meaning. Use\.to match a literal period. - Greedy matching:
.*matches too much. Use.*?for lazy matching. - Missing anchors:
\d{4}matches "2026" anywhere. Use^\d{4}$for exact match. - Overcomplicated patterns: Sometimes string methods or multiple simple patterns are clearer.
🔧 パターンをテストする
Use our free RegEx Tester to experiment with patterns and see matches in real-time.
RegEx テスターを開く →正規表現を使用しない場合
- Parsing HTML/XML: Use a proper parser. Regex can't handle nested tags correctly.
- Complex validation: Email RFC is incredibly complex. Use a library.
- Simple tasks:
str.includes()orstr.startsWith()are clearer than regex.
結論
Regular expressions are like a superpower—incredibly useful once you learn them, but easy to misuse. Start with simple patterns, test incrementally, and don't be afraid to use comments or break complex patterns into pieces.
The key to mastering regex is practice. Use the patterns in this tutorial as building blocks, experiment with variations, and soon you'll be writing patterns confidently.