How to match repeated characters using regular expression operator =~ in bash? - regex

Question

I want to know if a string has repeated letter 6 times or more, using the =~ operator.
a="aaaaaaazxc2"
if [[ $a =~ ([a-z])\1{5,} ]];
then
echo "repeated characters"
fi
The code above does not work.

Answer 1

BASH regex flavor i.e. ERE doesn't support backreference in regex. ksh93 and zsh support it though.
As an alternate solution, you can do it using extended regex option in grep:
a="aaaaaaazxc2"
grep -qE '([a-zA-Z])\1{5}' <<< "$a" && echo "repeated characters"
repeated characters
EDIT: Some ERE implementations support backreference as an extension. For example Ubuntu 14.04 supports it. See snippet below:
$> echo $BASH_VERSION
4.3.11(1)-release
$> a="aaaaaaazxc2"
$> re='([a-z])\1{5}'
$> [[ $a =~ $re ]] && echo "repeated characters"
repeated characters

Answer 2

[[ $var =~ $regex ]] parses a regular expression in POSIX ERE syntax.
See the POSIX regex standard, emphasis added:
BACKREF - Applicable only to basic regular expressions. The character string consisting of a character followed by a single-digit numeral, '1' to '9'.
Backreferences are not formally specified by the POSIX standard for ERE; thus, they are not guaranteed to be available (subject to platform-specific libc extensions) in bash's native regex syntax, thus mandating the use of external tools (awk, grep, etc).

Answer 3

You do not need the full power of backreferences for this specific case of one character repeats. You could just build the regex that would check for a repeat of every single lower case letter
regex="a{6}"
for x in {b..z} ; do regex="$regex|$x{6}" ; done
if [[ "$a" =~ ($regex) ]] ; then echo "repeated characters" ; fi
The regex built with the above for loop looks like
> echo "$regex" | fold -w60
a{6}|b{6}|c{6}|d{6}|e{6}|f{6}|g{6}|h{6}|i{6}|j{6}|k{6}|l{6}|
m{6}|n{6}|o{6}|p{6}|q{6}|r{6}|s{6}|t{6}|u{6}|v{6}|w{6}|x{6}|
y{6}|z{6}
This regular expression behaves as you would expect
> if [[ "abcdefghijkl" =~ ($regex) ]] ; then \
echo "repeated characters" ; else echo "no repeat detected" ; fi
no repeat detected
> if [[ "aabbbbbbbbbcc" =~ ($regex) ]] ; then \
echo "repeated characters" ; else echo "no repeat detected" ; fi
repeated characters
Updated following the comment from #sln replaced bound {6,} expression with a simple {6}.