Two Regular Expressions in one

[Peter Heide]

Regular Expressions can be used to validate if a String is in a valid format.

The following code checks if a string is valid using the following two rules

Starts with a letter from A to Z

Afterwards exactly 6 digits

Examples: A123456 is correct, A12345 is incorrect, AA is incorect

The Java code is as follows:

   
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class RegularExpression {
public static void validateID(String id) {
Pattern p = Pattern.compile("[A-Z]{1}[0-9]{6}\\z");
Matcher m = p.matcher(id);
boolean res = m.lookingAt();
System.out.println("ID " + id + " is " + res); 
}
public static void main(String[] args) {
validateID("A123456"); // true
validateID("A1"); // false
validateID("AA"); // false
}
}

Result Output:

ID A123456 is true
ID A1 is false
ID AA is false

The problem is that a new rule comes into play:

An ID is not valid when it starts with A33

How can I code the first two rules and the new one together in one regular expression?

[Anubhav Anand]

Try this

Pattern p = Pattern.compile("(?!A33)[A-Z]{1}[0-9]{6}\\z");

Visit this link. It has wonderful explanations.

[Included the regex article url]
[ April 03, 2008: Message edited by: Anubhav Anand ]

[Jim Yingst]

First a minor note - using lookingAt() is sort of an odd choice here, considering that you're also using \\z to force a match with the end of the pattern. It seems simpler to just use matches(), which requires a match of the entire expression, starting at the beginning and ending at, well, the end. Then you don't need to specify //z in the expression; it's implicit.

Anyway, to add your new rule to the regex, you can use negative lookahead:

Pattern p = Pattern.compile("(?!A33)[A-Z]{1}[0-9]{6}");

Basically the (?!A33) means from this position (the beginning, in this case) the matcher needs to be able to scan ahead and not see A33. Then when it gets to the ), the pattern can forget about the lookahead expression and remember it's still at the beginning of the expression, and try to match the remainder of the pattern (the A-Z]{1}[0-9]{6}).

[Peter Heide]

You solved the problem and also your advice to improve my code worked. Thank you very much Anubhav and Jim!