Regex required: It should contain only [alphabets] or [alphabets and digits] but not [only didgits] and not operator or special character in java

Question

Valid: abc abc11

Invalid: 11 a-b a&&b a << b a&b a->b

Answer 1

One regexp:

/^[a-zA-Z\d]*[a-zA-Z]+[a-zA-Z\d]*^/

Answer 2

You need two regexes. One that checks it's only alphanumeric /[a-zA-Z0-9]+/ and one that checks it's not just digits /^[^0-9]+$/ Only check the second one if the first one passes.

Answer 3

This regexp should do the job: /^[a-zA-Z\d]*[a-zA-Z][a-zA-Z\d]*^/

Mind you, this regex is potentially going to a significant amount of backtracking; it will be O(N^2) in the worst case. Making the first repetition lazy instead of eager will make the regex faster ... if it is going to match. The way to speed up the no-match case is to use two regexes.

Or better still, don't use regexes at all and code match in simple Java:

public boolean matchSymbol (String input) {
    boolean seenLetter = false;
    final int len = input.length();
    for (int i = 0; i < len; i++) {
        char ch = input.charAt(i);
        if (ch >= 'a' && ch <= 'z' || ch >= 'A' && ch <= 'Z') {
            seenLetter = true;
        } else if (ch < '0' || ch > '9') {
            return false;
        }
    }
    return seenLetter;
}

For an N character String, this succeeds in N loop iterations or fails in (typically) less than N iterations.