Regular Expressions are capable of describing the syntax of Tokens. Any syntactic construct that can be described by Regular Expression can also be described by the Context free grammar.

**Regular Expression:**

(a|b)(a|b|01)

**Context-free grammar:**

S --> aA|bA A --> aA|bA|0A|1A|e

*e denotes epsilon.

The Context-free grammar form NFA for the Regular Expression using the following construction rules:

- For each state there is a Non-Terminal symbol.
- If state A has a transition to state B on a symbol a
- IF state A goes to state B, input symbol is e
- If A is accepting state.
- Make the start symbol of the NFA with the start symbol of the grammar.

Every Regular set can be described the Context-free grammar that’s why we are using Regular Expression. There are several reasons and they are:

Regular Expressions | Context-free grammar |
---|---|

Lexical rules are quite simple in case of Regular Expressions. | Lexical rules are difficult in case of Context free grammar. |

Notations in regular expressions are easy to understand. | Notations in Context free grammar are quite complex. |

A set of string is defined in case of Regular Expressions. | In Context free grammar the language is defined by the collection of productions. |

It is easy to construct efficient recognizer from Regular Expressions. | By using the context free grammar, it is very difficult to construct the recognizer. |

There is proper procedure for lexical and syntactical analysis in case of Regular Expressions. | There is no specific guideline for lexical and syntactic analysis in case of Context free grammar. |

Regular Expressions are most useful for describing the structure of lexical construct such as identifiers, constant etc. | Context free grammars are most useful in describing the nested chain structure or syntactic structure such as balanced parenthesis, if else etc. and these can’t be define by Regular Expression. |

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