What is a regular expression that will match a character at certain indexes?

Question

Is there a single regular expression that will match strings that have the letter "a" or "x" at certain indexes?

For example, the indexes may be

0, 3, 6
1, 4, 7
2, 5, 8

Possible patterns according to these indexes:

"a--a--a--"
"-x--x--x-"
"--a--a--a"

Other occurrences of "a" or "x" are okay. I just want to make sure certain indexes contain "a".

EDIT: This problem is a subproblem of a tic-tac-toe board. I'm to determine if there is a winner by using a regular expression. I don't want the answer to this. I just want a bit of a push to help me move toward the final answer. The above is to help me find a regex that would identify if there is a vertical winner.

Is it always the same interval between the indexes? All your examples have them separated by 3. — Barmar, Dec 15 '16 at 19:54
If you know the indices then why not just check these 3 individual characters using simple `if` condition — anubhava, Dec 15 '16 at 19:55
The answer to this question will let me develop a more complicated regex, so I need to know how to do this with a regex — brienna, Dec 15 '16 at 19:56
"*The answer to this question will let me develop a more complicated regex*" this is recipe for [XY problem](http://meta.stackexchange.com/questions/66377/what-is-the-xy-problem). — Pshemo, Dec 15 '16 at 19:57
To be honest, regular expressions sound like a *terrible* fit for this problem. — NPE, Dec 15 '16 at 19:58
I understand that, but I really just want the solution to this part. — brienna, Dec 15 '16 at 19:58
Ok, I think answers will help you, if you want to go further, you can take a look at this question: http://stackoverflow.com/questions/17039670/vertical-regex-matching-in-an-ascii-image — Casimir et Hippolyte, Dec 15 '16 at 20:09

score 3 · Answer 1 · answered Dec 15 '16 at 20:14

3

If your tic-tac-toe looks like a 9 chars long string, testing this pattern should suffice to know if there's a vertical line:

a..a..a

answered Dec 15 '16 at 20:14

Casimir et Hippolyte

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anubhava · Accepted Answer · 2016-12-15T20:14:47.577

2

You can use this regex:

([a-zA-Z])..\1..\1

RegEx Demo

([a-zA-Z]) will match and capture any letter in group #`
Then another 2 instances of same letter at n+3 and n+6 positions are matched using back-reference \1, where n is the first position of letter.

edited Dec 15 '16 at 20:14

answered Dec 15 '16 at 19:57

anubhava

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He said `a` is allowed at other positions, it could be before the first one at the index he cares about. So you shouldn't have `[^a]*` to exclude those. – Barmar Dec 15 '16 at 20:13
1

I chose this answer because of its flexibility, cuz now I can test for any character(s). Thanks! – brienna Dec 15 '16 at 22:35

score 1 · Answer 3 · answered Dec 15 '16 at 19:57

1

Since a appears every 3 characters, (a..)* will match those repetitions. Then you just need to precede that with a pattern that matches the required number of characters before it, which is simply n-1 . patterns.

So for the three examples you gave, it's

^(a..)*
^.(a..)*
^..(a..)*

answered Dec 15 '16 at 19:57

Barmar

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Knowing he always matches this on a 9-char string, why not simply `(a..){3}` ? – JM Lord Dec 15 '16 at 20:17
I thought he wanted to find the match starting at a specific position. – Barmar Dec 15 '16 at 20:18
You're right, the regex I gave works only for the problem he gave (verticals), but would not work for the diagonals of the tic-tac-toe. Anyways, this matters not : @Casimir et Hippolyte 's answer is right on, and extremely simple. – JM Lord Dec 15 '16 at 20:21

What is a regular expression that will match a character at certain indexes?

3 Answers3