Regular Expression, words with different characters

Question

I have been searching about it for about 3 hrs, now I'm stuck. Here is the problem:

I want to find 7 character word where only 4. and 6. chars are same

like:

^...(.).\1.$

but i don't want to find

aaaBaBa

all other chars must different than each other like:

asdBfBg

my question is similar to this one: Java: Regular expression where each character occurs 0-1 times

but my knowledge wasn't enough to work with Lookarounds (?<= and (?=, (?

Could you give some more sample strings for what you want your regex to match on, and what you don't want it to match on? — Meredith, Aug 21 '13 at 00:54

p.s.w.g · Answer 1 · 2013-08-21T01:00:30.460

1

Perhaps you can use a negative assertion, like this:

^(.)(.)(.)(.)(.)\4(.)(?<!(?:\1.*\1|\2.*\2|\3.*\3|\4.*\4.*\4|\5.*\5|\6.*\6).*)$

This will find any 7-letter word with the 4th and 6th letters the same, and whose 1st, 2nd, 3rd, 5th, or 7th (\6) letter do not appear twice, and whose 4th letter is not repeated three times.

edited Aug 21 '13 at 01:00

answered Aug 21 '13 at 00:55

p.s.w.g

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Did you try this? It seems off and I don't know if many regex engines allow back-references in lookbehinds. – pguardiario Aug 21 '13 at 01:10
@pguardiario Yes, I tested it and it does work using the .NET engine. – p.s.w.g Aug 21 '13 at 01:25
I keep getting errors on visual regexp it gives me **"quantifier operand invalid"** and on bash with using grep it says **"Invalid back reference"** – Irfan Bilaloğlu Aug 21 '13 at 12:35

score 1 · Answer 2 · answered Aug 21 '13 at 01:14

1

Here's something that works, maybe there's a better way to do this though.

/
\b # word boundary
(\w) # any word char
(?!\1)(\w) # any word char except \1
(?!\1|\2)(\w) # any word char except \1 or \2
(?!\1|\2|\3)(\w) # etc...
(?!\1|\2|\3|\4)(\w)
\4 # 4th capture
(?!\1|\2|\3|\4\5)(\w)
\b # word boundary
/x

answered Aug 21 '13 at 01:14

pguardiario

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did you try this because on grep i get **"Invalid back reference"** – Irfan Bilaloğlu Aug 21 '13 at 14:10
This is a perl-style regex. I don't believe that grep is sophisticated enough for something like this. – pguardiario Aug 21 '13 at 22:46

Regular Expression, words with different characters

2 Answers2