Regular expression for odd number a

Question

I had a problem resolving the following exercise, and I would appreciate any help.

Let Σ = {a, b}. I need to give a regex for all lines containing an odd number.

thank you for your time

+4

kskyriacou Mar 6 '15 at 15:53

2 answers

^[^a]*a(?=[^a]*(?:a[^a]*a)*[^a]*$).*$

a's . . .

0

vks 06 . '15 16:08

HuStmpHrrr · Accepted Answer · 2015-03-06T16:05:46+0000

b*(ab*ab*)*ab*

its main part (ab*ab*)*, which lists all the possibilities of an even number as. then finally an extra amust exist to make it odd.

note that this regular expression is equivalent to:

b*a(b*ab*a)*b*

these two constructions are in the form determined by the pump lemma:

UPDATE:

@MahanteshMAmbi , aaabaaa. . grep, , .

$ echo aaabaaa | grep -P -o 'b*(ab*ab*)*ab*'
aaabaa
a

-o grep . , , . 5 a s, 1 a. , .

, , :

^b*(ab*ab*)*ab*$

:

$ echo aaabaaa | grep -P -q '^b*(ab*ab*)*ab*$'
$ echo $?
1