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A context free grammar (CFG) is a forma grammar which is used to generate all the possible patterns of strings in a given formal language.

It is defined as four tuples −

**G=(V,T,P,S)**

- G is a grammar, which consists of a set of production rules. It is used to generate the strings of a language.
- T is the final set of terminal symbols. It is denoted by lower case letters.
- V is the final set of non-terminal symbols. It is denoted by capital letters
- P is a set of production rules, which is used for replacing non-terminal symbols (on the left side of production) in a string with other terminals (on the right side of production).
- S is the start symbol used to derive the string

Construct CFG for the language having any number of a's over the set ∑={a}

Regular Expression= a*

Production rule for the Regular Expression is as follows −

S->aS rule 1

S-> ε rule 2

Now if we want to derive a string "aaaaaa" we can start with start symbol

Start with start symbol:

s | rule |

aS | 1 |

aaS | 1 |

aaaS | 1 |

aaaaS | 1 |

aaaaaS | 1 |

aaaaaaS | 1 |

aaaaaa | 2 |

The regular expression=a* can generate a set of strings { ε,a,aa,aaa,...}

We can have a null string because S is a start symbol and rule 2 gives S-> ε

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