Why Does Java\'s `String.matches()` Fail to Find Lowercase Words?

Regex Puzzle: Why Does String.matches() Misbehave?

In the following code snippet, the objective is to use regular expressions to identify words that contain only lowercase letters:

String[] words = {"{apf","hum_","dkoe","12f"};
for(String s:words)
{
    if(s.matches("[a-z]"))
    {
        System.out.println(s);
    }
}

However, instead of printing "dkoe," which is the expected result, the code produces no output. Why is this happening?

The Pitfalls of String.matches()

The culprit lies in the misunderstanding of the String.matches() method. Unlike its seemingly similar counterparts in other languages, matches() in Java attempts to match the entire input string against the provided regular expression. This means it requires the expression to match the entire string, not just a portion of it.

In the given code, the regular expression [a-z] matches any single lowercase letter. Thus, for matches() to return true, the entire input string must consist of exactly one lowercase character. However, none of the input words satisfy this condition.

The Solution: Introducing Pattern and Matcher

To resolve this issue, it's recommended to use Pattern and Matcher objects, which offer more control and flexibility in regex matching:

Pattern p = Pattern.compile("[a-z]+");
Matcher m = p.matcher(inputstring);
if (m.find())
    // match found

By using the quantifier within the character class, we are ensuring that the expression matches strings containing one or more lowercase characters, which will correctly identify the word "dkoe."

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