Question

Suppose if I wanted to extract the suffix string “aabdcd” from the string daaaaab*aabdcd* , the following expression is used in KSH,

${ex##+([!b])+([b])}

This means match and remove any character other than ‘b’ greedily followed by the character ‘b’. What I don’t understand is the + (highlighted in bold) preceding. Why is it required? Without it the regex fails to do the intended job.

#ex=daaaaabaabdcd
#echo ${ex##([!b])+([b])}
daaaaabaabdcd
#echo ${ex##+([!b])+([b])}
aabdcd

Answer 1

The + represents the regex quantifier of the result. So with the + you may receive multiple results. Here are the other quantifiers that can be used:

?(pattern) matches zero or one times the pattern.
*(pattern) matches any time the pattern.
+(pattern) matches one or more time the pattern.
@(pattern) matches one time the pattern.
!(pattern) matches string without the pattern.