bash - Regex collating symbols -

i tried understand how 'collating symbols' match works did not come out this. understood means matching exact sequence instead of character(s), is:

echo "ciiiao" | grep '[oa]'     --> output 'ciiiao' echo "ciiiao" | grep '[[.oa.]]' --> no output echo "ciiiao" | grep '[[.ia.]]' --> output 'ciiiao' 

however, third command not work. wrong or misinterpret something?

i have read regexp tutorial.

collating symbols typically used when digraph treated single character in language. element of posix regular expression specification, , not supported.

for example, in languages "sh" treated single character. assuming locale file defines (a collating symbol work if defined in current locale), collating symbol [[.sh.]] treated single character. likewise, searching . or [^a] match "sh." if locale defined "sh" following "h" in alphabet, [g-i] match "sh."