12 Oct 2004 00:55:43 -0400

Related articles |
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help with regular expressions johndoe@yahoo.com (keith) (2004-10-09) |

Re: help with regular expressions cdc@maxnet.co.nz (Carl Cerecke) (2004-10-12) |

Re: help with regular expressions genew@mail.ocis.net (Gene Wirchenko) (2004-10-12) |

Re: help with regular expressions jeremy.wright@microfocus.com (Jeremy Wright) (2004-10-12) |

Re: help with regular expressions torbenm@diku.dk (2004-10-12) |

From: | torbenm@diku.dk (Torben Ęgidius Mogensen) |

Newsgroups: | comp.compilers |

Date: | 12 Oct 2004 00:55:43 -0400 |

Organization: | Department of Computer Science, University of Copenhagen |

References: | 04-10-077 |

Keywords: | lex |

Posted-Date: | 12 Oct 2004 00:55:43 EDT |

keith <johndoe@yahoo.com> writes:

*> write the regular expression for strings over the alphabet {a,b,c}*

*> that don't contain the contiguous substring baa. I've come up with*

*> this:*

*>*

*> (a|c|(ba(b|c))|bb|bc)**

*>*

*> ??? Is this correct?*

No, the strings "bbaa" and "babaa" are in the language you describe.

*> binary numbers n such that there exists an integer solution of an+bn=cn,*

*> don't know what to do about this one :-( As far as I am concerned all*

*> binary numbers satisfy this condition as integer+*integer=integer.*

The exercise says a^n+b^n=c^n, for which Fermats theorem says that

there exist integer solutions only when 0<=n<=2.

Torben

[I have an elegant regular expression that solves the problem, but

the margin of this message is too small to contain it.

-John]

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