java 26 lines · 7 steps

Converting camelCase to snake_case in Java

A regex that spots case boundaries turns camelCase identifiers into snake_case with underscores.

Explained by highlit

Intermediate regex string-manipulation utility-class immutability

1public final class CaseConverter {
2 
  private static final Pattern CAMEL_BOUNDARY =
          Pattern.compile("([a-z0-9])([A-Z])|([A-Z]+)([A-Z][a-z])");
5 
  private CaseConverter() {
  }
8 
  public static String toSnakeCase(String camelCase) {
      if (camelCase == null || camelCase.isEmpty()) {
          return camelCase;
      }
13 
      Matcher matcher = CAMEL_BOUNDARY.matcher(camelCase);
      StringBuilder result = new StringBuilder();
      while (matcher.find()) {
          String replacement = matcher.group(1) != null
                  ? matcher.group(1) + "_" + matcher.group(2)
                  : matcher.group(3) + "_" + matcher.group(4);
          matcher.appendReplacement(result, Matcher.quoteReplacement(replacement));
      }
      matcher.appendTail(result);
23 
      return result.toString().toLowerCase(Locale.ROOT);
  }
26}

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Compiling a Pattern once as a static constant avoids re-parsing the regex on every call.
2Two alternations let a single regex handle both normal boundaries and acronym-to-word transitions.
3appendReplacement plus appendTail rebuild a string while touching only the matched regions.

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