Regular expression example question.

[[:alpha:]] Means a set of alphabetic characters and matches any alphabetic character.

+ The number of appearances operator, 1..It means more than one or more.

? The number of appearances operator, which means 0..1 (0 or 1).

( ) Regular expression matching groups.

A. In order of interpret it in order.

[[:alpha:]]

B. Any alphabetic character.

[[:alpha:]]+

C. String consisting of one or more consecutive alphabets.

[[:alpha:]]+ ?

D. After a string of one or more consecutive alphabets appears, a space may appear or may not appear.

([[:alpha:]]+ ?)+

The preceding pattern D must be a string that repeats at least once.

Therefore, for convenience, the following string will be expressed as _(underbar).)

aa_bb_ccc

D appears 3 times to satisfy the regular expression.

aa_bb_ccc_

It is also acceptable if there are more blank characters as shown above.