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javaregexunit-testingpatternsyntaxexception

Why is no PatternSyntaxException thrown for my invalid pattern?

I want to test whether a regexp is valid in Java 1.8.0_241

public static boolean isRegExpValid(String regExp) {
    try {
        Pattern.compile(regExp);
        return true;
    } catch (PatternSyntaxException e) {
        return false;
    }
}

Here I am testing a correct regexp for a three digit number, and an incorrect regexp.

@Test
public void testValidRegexp() {
    assertTrue(isRegExpValid("\\d{3}"));
}

@Test
public void testInvalidRegexp() {
    assertFalse(isRegExpValid("{3}"));
}

Why is my second test testInvalidRegexp failing? isRegExpValid("{3}") should return false, but returns true.

In Javascript, {3} correctly fails with a Nothing to repeat exception. enter image description here

Solution

  • It appears to successfully match the empty string:

    jshell> Pattern.matches("{3}x", "x")
$1 ==> true

    The documentation doesn't appear to define this as a valid use, but it passes where other minor syntax problems would be detected and throw Pattern.error.

    To analyse why the implementation accepts this, you'd want to figure out where Pattern#closure can be called from while parsing a pattern.