正規表現を使う方法

问题

単純な文字列メソッドでは処理できないテキストパターンを検索・検証する必要があります。

前提条件

• テキストパターンの基礎理解
• 特殊文字への習熟

ステップ

1. 1. Learn the core syntax

   Regex uses literals to match exact characters and metacharacters for patterns. The dot (.) matches any character, * means zero or more, + means one or more, ? means optional, and [] defines a character class. Anchors ^ and $ match the start and end of the string.

   入力

   pattern: ^\d{3}-\d{4}$

   输出

   matches: 123-4567

2. 2. Build and test the pattern

   Construct the regex for your use case. Test it against matching and non-matching strings. Use a regex tester with explanation to understand how the engine matches. Pay attention to greediness (* and + are greedy by default; add ? for lazy matching).

   入力

   pattern: <.+?>

   输出

   lazy match: <b> (not <b>text</b>)

3. 3. Use capture groups and flags

   Parentheses create capture groups for extraction. Use (?:...) for non-capturing groups. Flags modify behavior: i for case-insensitive, g for global (all matches), m for multiline. Backreferences (\1) refer to previously captured groups.

   入力

   (\w+)@\w+

   输出

   captures the name part of an email

期待される結果

目的のテキストを正しくマッチ、検証、抽出できる、動作する正規表現パターン。

よくあるエラー

Pattern matches too much (greedy)

Add ? after * or + to make the quantifier lazy, or use more specific character classes.

Special characters not escaped

Escape regex metacharacters (.*+?^${}()|[]\) with a backslash when you want to match them literally.

トラブルシューティング指引

• If the pattern does not match, test it in a regex debugger that shows step-by-step matching.
• Catastrophic backtracking can cause exponential slowdown. Avoid nested quantifiers like (a+)+.

セキュリティ上の制限

• Regex denial of service (ReDoS) is a real threat. Avoid patterns with potential for catastrophic backtracking on untrusted input.