Incorporate egrep regexps with awk? - regex

Question

I've been trying to understand how awk can work with egrep regular expressions.
I have the following example:
John,Milanos
Anne,Silverwood
Tina,Fastman
Adrian,Thomassonn
I'm looking to use egrep regexps to process the second column (the last names in this scenario) while printing the entire line for output.
The closest I've come to my answer was using?
$ awk -F ',' '{print $2}' | egrep '([a-z])\1.*([a-z])\2'
Thomassonn
I would then take "Thomassonn" and egrep back into my initial list of full names to get the full record. However, I've encountered plenty of errors and false positives once I used other filters.
Desired result:
Adrian,Thommasson

Answer 1

awk does not support back-references within a regex. egrep, however, is sufficient to achieve your desired result:
$ egrep ',.*([a-z])\1.*([a-z])\2' file
Adrian,Thomassonn
Variations
If there are three or more columns and you want to search only the second:
egrep '^[^,]*,[^,]*([a-z])\1[^,]*([a-z])\2' file
If you want to search the third column:
egrep '^[^,]*,[^,]*,[^,]*([a-z])\1[^,]*([a-z])\2' file
If you want to search the first of any number of columns:
egrep '^[^,]*([a-z])\1[^,]*([a-z])\2' file

Answer 2

awk doesn't support backreferences, here's one way to do what you want instead:
$ cat tst.awk
BEGIN{ FS="," }
{
numMatches = 0
fld = $2
for (charNr=1; charNr <= length($2); charNr++) {
char = substr($2,charNr,1)
if (char ~ /[a-z]/)
numMatches += gsub(char"{2}"," ",fld)
}
}
numMatches >= 2
$
$ awk -f tst.awk file
Adrian,Thomassonn
If you want to match sequences of 3 or any other number of repeated chars, just change {2} to {3} or whatever number you like.
By the way, for portability to all locales you should use [[:lower:]] instead of [a-z] if that's what you really mean.