Regex must contain specific letters in any order

Question

I have been attempting to validate a string in VB.net that must contain these three letters in no particular order and do not need to be next to One another. ABC

I can do this easily using LINQ

MessageBox.Show(("ABC").All(Function(n) ("AAAABBBBBCCCC").Contains(n)).ToString)

However, after searching Google and SO for over a week, I am completely stumped. My closest pattern is ".*[A|B|C]+.*[A|B|C]+.*[A|B|C]+.*" how ever AAA would also return true. I know i can do this using other methods just after trying for a week i really want to know if its possible using One regular expression.

Answer 1

Your original pattern won't work because it will match any number of characters, followed by one or more A, B, C, or | character, followed by any number of characters, followed by one or more A, B, C, or | character, followed by any number of characters, followed by one or more A, B, C, or | character, followed by any number of characters.

I'd probably go with the code you've already written, but if you really want to use a regular expression, you can use a series of lookahead assertions, like this:

(?=.*A)(?=.*B)(?=.*C)

This will match any string that contains A, B, and C in any order.

Answer 2

You can use zero-width lookaheads. Lookaheads are great to eliminate match possibilities if they don't meet a certain criteria.

For example, let's use the words

untie queue unique block unity

Start with a basic word match:

\b\w+\b

to require the word matched with \w+ begins with un, we could use a positive lookahead

\b(?=un)\w+\b

What this says is

• \b Match a blank
• (?=un) Are there the letters "un"? If not, NO MATCH. If so, then possible match.
• \w+ One or more word characters
• \b Match a blank

A positive lookahead eliminates a match possibility if it does NOT meet the expression inside. It applies to the regex RIGHT AFTER it. So the (?=un) applies to the \w+ expression above and requires that it BEGINS WITH un. If it does not, then the \w+ expression won't match.

How about matching any words that do not begin with un? Simply use a "negative lookahead"

\b(?!un)\w+\b

• \b Match a blank
• (?!un) Are there the letters "un"? If SO, NO MATCH. If not, then possible match.
• \w+ One or more word characters
• \b Match a blank

So for your requirement of having at least 1 A, 1 B and 1 C in the string, a pattern like

(?=.*A)(?=.*B)(?=.*C).+

Works because it says:

• (?=.*A) - Does it have .* any characters followed by A? If so, possible match if not no match.
• (?=.*B) - Does it have .* any characters followed by B? If so, possible match if not no match.
• (?=.*C) - Does it have .* any characters followed by C? If so, possible match if not no match.
• .+ If the above 3 lookahead requirements were met, match any characters. If not, then match no characters (and so there isn't a match)

Answer 3

Does it have to be a regex? That's something that can easily be solved without one.

I've never programmed in VB, but I'm sure there are helper functions that let you take a string, and query whether or not a character occurs in it.

If str is your string, maybe something like:

str.contains('A') && str.contains('B') && str.contains('C')

Answer 4

You can make use of positive lookaheads:

^(?=.*A)(?=.*B)(?=.*C).+

(?=.*A) makes sure there's an A somewhere in the string and the same logic applies to the other lookaheads.