JAVA regex pattern if statement

Question

This is what i have already.

^[abceghj-prstw-z][a-np-z]$

I am trying to form regex pattern with these requirements: First position can be any letter but d,f,i,q,u,v. Second position can be any letter but o. The first and second position can't be BG, GB, NK, KN, TN, NT, ZZ.

So for example string "ap" = true.

ao = false (because second position is o).

gb = false (because it cant be gb)

I am pretty new with regular expressions so any help would be great. Thanks.

Please show some example input and whether it should match or not. — steffen, Jul 19 '19 at 11:55
It is very unclear what you are asking. Are you trying to form the correct regex, or are you wanting to know how to use this regex in Java. Please try to make your question more clear — tjc, Jul 19 '19 at 11:59
some *candy*: you can use `[a-z&&[^dfiqv]]` to match any character between 'a' and 'z' with 'd','f','i',q'','v' excluded (but not that sure if that is more readable) — user85421, Jul 19 '19 at 19:24

MonkeyZeus · Accepted Answer · 2019-07-19T12:19:18.967

1

You need to make use of negative lookahead to make the regex fail if those specific patterns exist:

^(?i)(?!(bg)|(gb)|(nk)|(kn)|(tn)|(nt)|(zz))[abceghj-prstw-z][a-np-z]$

(?i) makes it case-insensitive.

edited Jul 19 '19 at 12:19

answered Jul 19 '19 at 12:13

MonkeyZeus

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score 0 · Answer 2 · answered Jul 19 '19 at 12:13

0

As answered here you may add negative lookahead to exclude forbidden symbols from the beginnig of your regex:

^(?!bg|gb|nk|kn|tn|nt|zz)[abceghj-prstw-z][a-np-z]$

answered Jul 19 '19 at 12:13

Jakub Ch.

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score 0 · Answer 3 · answered Jul 19 '19 at 12:17

You can use negative lookaheads or negative lookbehinds if you don't want to check the exceptions (gb, ...) manually. Here's an example with a negative lookbehind:

Pattern p = Pattern.compile("[abceghj-prstw-z][a-np-z](?<!gb|bg|nk|kn|tn|nt|zz)", Pattern.CASE_INSENSITIVE);
List<String> inputs = Arrays.asList("ap", "apo", "AP", "GB", "gb", "gg");
for (String input : inputs) {
    System.out.println(input + " " + p.matcher(input).matches());
}

Prints:

ap true
apo false
AP true
GB false
gb false
gg true

JAVA regex pattern if statement

3 Answers3