LicenseHeaderStep more permissive with parsing existing years

[nedtwigg]

Seems like a better behavior, based on user's experience in #677.

This PR is all just pure refactor, except this one commit: fbb3dc9

[nedtwigg]

And here is the logic for pulling years out of the existing header.

spotless/lib/src/main/java/com/diffplug/spotless/generic/LicenseHeaderStep.java

Line 238 in fbb3dc9

private static final Pattern YYYY = Pattern.compile("[0-9]{4}");

spotless/lib/src/main/java/com/diffplug/spotless/generic/LicenseHeaderStep.java

Lines 256 to 275 in fbb3dc9

	/** Searches the given string for YYYY, and uses that to determine the year range. */
	private String calculateYearBySearching(String content) {
	Matcher yearMatcher = YYYY.matcher(content);
	if (yearMatcher.find()) {
	String firstYear = yearMatcher.group();
	String secondYear;
	if (updateYearWithLatest) {
	secondYear = firstYear.equals(yearToday) ? null : yearToday;
	} else if (yearMatcher.find(yearMatcher.end() + 1)) {
	secondYear = yearMatcher.group();
	} else {
	secondYear = null;
	}
	return secondYear == null ? firstYear : firstYear + yearSepOrFull + secondYear;
	} else {
	System.err.println("Can't parse copyright year '" + content + "', defaulting to " + yearToday);
	// couldn't recognize the year format
	return yearToday;
	}
	}

[nedtwigg]

Released in plugin-gradle 5.5.0 and plugin-maven 2.3.0.