Replace character sequence with several characters using AWK gsub() - awk

Question

I'm trying to convert text by replacing several identical letters (more than 3) with several characters (two *).
My input:
ffffOOOOuuuurrrr
fffffiiiiivvvvveeeee
What should I get:
**OOOO****
********
My test command is:
awk '{gsub(/[a-z]{4}/,"*"); print}' textfile
I don't understand how to transform {4} into 'more than 3'.
Also how to print * two times (like multiply it).
I'm also sure that the condition 'more than three' will convert input into:
**OOOO**
**
Is there any way to avoid this (replace a sequence of identical letters)?
Or it is not possible to fit in one small command.

Answer 1

POSIX awk or sed don't support back-references. You will need to use gnu-sed or perl:
sed -E 's/([a-z])\1{3,}/**/g' file
**OOOO****
********
or using perl:
perl -pe 's/([a-z])\1{3,}/**/g' file
RegEx Details:
([a-z]): Match [a-z] and capture in group #1
\1: Back-reference of the letter captured in group #1
{3,}: Repeat 3 or more times

Answer 2

You mentioned sed as an option in the tags:
echo "fffffiiiiivvvvveeeee" | sed 's/\([A-Za-z]\)\1\1\1\+/\1/g'
five
echo "fffffiiiiivvveeeee" | sed 's/\([A-Za-z]\)\1\1\1\+/\1/g'
fivvve

Answer 3

Here's how to do it with any awk assuming a locale where lower case letters are a-z = ASCII 97-122:
$ cat tst.awk
{
for (i=97; i<122; i++) {
gsub(sprintf("%c{3,}",i),"**")
}
print
}
$ awk -f tst.awk file
**OOOO****
********
otherwise with GNU awk for the ord() function:
$ cat tst.awk
#load "ordchr"
{
for (i=ord("a"); i<=ord("z"); i++) {
gsub(sprintf("%c{3,}",i),"**")
}
print
}
$ awk -f tst.awk file
**OOOO****
********
or iyou can use a different numeric loop range or split("abc...z",...) or whatever else to get the loop but the point is - you need to loop on each character.