import java.util.regex.Matcher;
import java.util.regex.Pattern;
 
/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		final String regex = "(?:^\\W*|\\b(?<!vencimiento)\\W+)([0-3]?\\d[-/][01]?\\d[-/]\\d{4}\\b)";
		final String texto = "fecha vencimiento 20-12-2017\nfecha 20-12-2017\notra: 12/05/2016";
 
		final Pattern pattern = Pattern.compile(regex);
		final Matcher matcher = pattern.matcher(texto);
 
		if (matcher.find()) {
		    do {
		        System.out.println("Fecha: " + matcher.group(1));
		    } while (matcher.find());
		} else {
		    System.out.println("No se encontró ninguna fecha");
		}
	}
}

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