# Converting an NFA to an equivalent Regular Expression – the GNFA method

The GNFA method is a procedure to convert any given NFA to an equivalent regular expression. The idea is to rip out one state at a time, while modifying the transitions to reflect the changes. Here is an example explaining the process in detail

## NFA

Consider the following NFA

This can be expressed as the following state transition diagram

## Adding start and final states

The first step is to add a new start and a single new final state. The new start state is labeled as . The new start state has an transition to the original start state of the NFA. The new final state is labeled as . All the final state(s) in the original NFA have an transition to the new final state. The two new additions are marked in color.

Every state except and needs to ripped out. The order of choosing a state to rip out doesn’t really matter.

## Rip

Consider the state . The incoming (blue), looping (orange) and outgoing (green) transitions for this state are marked in different colors.

Thus, there are two paths through . This is summarized below

To rip state , the transitions along these paths need to accounted for. The equivalent transition is defined as



Applying this to the table above

After ripping the state the NFA changes to the following

## Rip

Consider the state . The incoming (blue), looping (orange) and outgoing (green) transitions for this state are marked in different colors.

Thus, there are two paths through . This is summarized below

To rip state , the transitions along these paths need to accounted for. The equivalent transition is defined as



Applying this to the table above

After ripping the state the NFA changes to the following

## Rip

Consider the state . The incoming (blue), looping (orange) and outgoing (green) transitions for this state are marked in different colors.

Thus, there is one path through . This is summarized below

To rip state , the transitions along these paths need to accounted for. The equivalent transition is defined as



Applying this to the table above

After ripping the state the NFA changes to the following

## Regular Expression

The equivalent regular expression is the transition from to .



Depending on the order of ripping out states, the NFA may be converted to different equivalent regular expressions.