ERE - adding quantifier to group with inner group and back-reference

Question

Was trying to get words with consecutive repeated letters occurring twice or thrice. Not able find a way to use quantifier and capture group using ERE

$ grep --version | head -n1
grep (GNU grep) 2.25

$ # consecutive repeated letters occurring twice
$ grep -m5 -xiE '[a-z]*([a-z])\1[a-z]*[a-z]*([a-z])\2[a-z]*' /usr/share/dict/words
Abbott
Annabelle
Annette
Appaloosa
Appleseed

$ # no output for this, why?
$ grep -m5 -xiE '([a-z]*([a-z])\2[a-z]*){2}' /usr/share/dict/words

Works with -P though

$ grep -m5 -xiP '([a-z]*([a-z])\2[a-z]*){2}' /usr/share/dict/words
Abbott
Annabelle
Annette
Appaloosa
Appleseed

$ grep -m5 -xiP '([a-z]*([a-z])\2[a-z]*){3}' /usr/share/dict/words
Chattahoochee
McConnell
Mississippi
Mississippian
Mississippians

Thanks Casimir et Hippolyte for coming up with simpler input and regex to test this behavior

$ echo 'aazbb' | grep -E '(([a-z])\2[a-z]*){2}' || echo 'No match'
aazbb
$ echo 'aazbbycc' | grep -E '(([a-z])\2[a-z]*){2}([a-z])\3[a-z]*' || echo 'No match'
aazbbycc
$ echo 'aazbbycc' | grep -P '(([a-z])\2[a-z]*){3}' || echo 'No match'
aazbbycc

$ # failing case
$ echo 'aazbbycc' | grep -E '(([a-z])\2[a-z]*){3}' || echo 'No match'
No match

Same behavior seen with sed as well

$ sed --version | head -n1
sed (GNU sed) 4.2.2

$ echo 'aazbb' | sed -E '/(([a-z])\2[a-z]*){2}/! s/.*/No match/'
aazbb    
$ echo 'aazbbycc' | sed -E '/(([a-z])\2[a-z]*){2}([a-z])\3[a-z]*/! s/.*/No match/'
aazbbycc

$ # failing case
$ echo 'aazbbycc' | sed -E '/(([a-z])\2[a-z]*){3}/! s/.*/No match/'
No match

Related search links, I checked some of them, but didn't get anything close to this question

• https://savannah.gnu.org/bugs/?group=grep
• http://lists.gnu.org/archive/html/bug-sed/

If this is solved in newer version of grep or sed, let me know. Also, if the issue is seen in non-GNU implementations

Answer 1

I suppose -E doesn't allow Quantifiers, that's why it works only with -P

---

to match 2 or more consecutive groups of repeated letters:

grep -P '(?:([a-z])\1*([a-z])\2){1}' /usr/share/dict/words

to match 3 or more consecutive groups of repeated letters:

grep -P '(?:([a-z])\1*([a-z])\2){2}' /usr/share/dict/words

---

Options:

-P, --perl-regexp         PATTERN is a Perl regular expression