Java Regular Expressions with Pattern and Matcher

Java regular expressions are compiled into a Pattern and applied through a Matcher.

Pattern pattern = Pattern.compile("(\\d{4})-(\\d{2})-(\\d{2})");
Matcher matcher = pattern.matcher("date=2026-07-16");

if (matcher.find()) {
    System.out.println(matcher.group(0));
    System.out.println(matcher.group(1));
}

Use matches() when the complete input must match, find() to locate subsequences, and lookingAt() to match from the beginning. Named groups improve readability:

Pattern.compile("(?<year>\\d{4})-(?<month>\\d{2})-(?<day>\\d{2})");

Java string literals require their own escaping, so a regex backslash is usually written as \\. Use Pattern.quote for literal input and Matcher.quoteReplacement for literal replacement values.

Compile frequently reused patterns once. Avoid nested ambiguous quantifiers that can cause catastrophic backtracking on adversarial input. Apply input-size limits and prefer explicit parsers for complex structured formats.