Java split and contains: Regex Escaping and Empty Fields

Some Java string APIs accept literal text, while others accept regular expressions. Similar names can conceal different argument semantics. A dot used as a split delimiter is a particularly common source of surprises.

contains searches text; split uses a regex

String value = "aaa.add.d";
System.out.println(value.contains("."));       // true
System.out.println(value.split(".").length);  // 0
System.out.println(value.split("\\.").length); // 3

In Java source, the final delimiter must be "\\.": Java string parsing produces a backslash and dot, then the regex parser interprets them as a literal dot. The regex . matches any character. Since split discards trailing empty strings by default, the second result has length zero.

Use Pattern.quote to avoid manually escaping a literal delimiter:

String[] parts = value.split(java.util.regex.Pattern.quote("."));

Decide whether empty fields matter

System.out.println("a,b,".split(",").length);     // 2
System.out.println("a,b,".split(",", -1).length); // 3

A negative limit preserves trailing empty fields, useful in simple delimited text where position matters. Real CSV includes quoting, escaping and line breaks inside fields, so one split cannot parse it correctly.

Compare similar methods

Dollar signs and backslashes in replacements can also be interpreted specially. If externally supplied replacement content must remain literal, use Matcher.quoteReplacement. Decide whether regex behavior is needed before choosing the API; that is usually clearer than repeatedly adding backslashes.

References

• Java String
• Pattern