Regular expression for language

Question

Let's have a language $L=\{\omega\in\{a,b,c\}^* | \omega $ contains $ab$ and does not contain $ba\}$ make a regular expression for this language.

I've ended up with this one $$(a^*(b^*+(cc^{*}a^*))^*)^*ab(b^*+(cc^{*}a^*))^*$$

Is this correct? Does it generate all words containing $ab$ and not containing $ba$ ?

dfeuer · Accepted Answer · 2013-11-02T15:54:59.047

2

This is incorrect. It accepts $bab=(a^0(b^1)^1)^1ab$. In the first part of the expression, you want to tie $b$ up to $c$, not $a$ to $c$.

edited Nov 02 '13 at 15:54

answered Nov 02 '13 at 15:40

dfeuer

1

$(a^(b^+(cc^{}a^))^*)^0ab(b^1)^1 = abb$ – Noturab Nov 02 '13 at 15:41
@DávidKaya How do you get that $b^1$ without $c$? Did you mean to have $(cc^a^)^*$? – dfeuer Nov 02 '13 at 15:45
there's $+$ in $(b^+(cc^{}a^))^$ – Noturab Nov 02 '13 at 15:45
@DávidKaya, I forgot what that $+$ meant. Fixed now. – dfeuer Nov 02 '13 at 15:49
Yea, that was my mistake... If I change the regular expression in my question, will you be ok with that, or do you want me to accept your question? – Noturab Nov 02 '13 at 15:57
@DávidKaya, you should add an answer of your own with a corrected version. – dfeuer Nov 02 '13 at 16:06

1 Answers1