Regular expressions for token patterns in Compiler Design - Step-by-Step Execution

Concept Flow - Regular expressions for token patterns

Start

↓

Input string

↓

Apply regex patterns

↓

Match token pattern?

No→Error or skip

Yes↓

Extract token

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Store token

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More input?

Yes→Apply regex patterns

No↓

End

The process starts with an input string, applies regex patterns to find tokens, extracts and stores them, and repeats until all input is processed.

Execution Sample

Compiler Design

Input: "var x = 42;"
Patterns: identifier=[a-zA-Z_][a-zA-Z0-9_]*
          number=[0-9]+

This example shows how regex patterns match tokens like identifiers and numbers in a simple code snippet.

Analysis Table

Step	Input Segment	Regex Pattern	Match Result	Token Extracted	Next Action
1	"var x = 42;"	identifier=[a-zA-Z_][a-zA-Z0-9_]*	Matches 'var'	Token: identifier='var'	Store token, move forward
2	" x = 42;"	identifier=[a-zA-Z_][a-zA-Z0-9_]*	Matches 'x'	Token: identifier='x'	Store token, move forward
3	" = 42;"	identifier=[a-zA-Z_][a-zA-Z0-9_]*	No match	-	Try next pattern
4	" = 42;"	number=[0-9]+	No match	-	Try next pattern or skip
5	" = 42;"	symbol '='	Matches '='	Token: symbol='='	Store token, move forward
6	" 42;"	number=[0-9]+	Matches '42'	Token: number='42'	Store token, move forward
7	" ;"	symbol ';'	Matches ';'	Token: symbol=';'	Store token, move forward
8	" "	-	-	End of input reached	-

💡 All input processed, no more tokens to extract.

State Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	Final
Input String	"var x = 42;"	" x = 42;"	" = 42;"	" = 42;"	" = 42;"	" 42;"	" ;"	" "	""
Current Token	-	'var'	'x'	-	-	'='	'42'	';'	-
Tokens List	[]	['var']	['var', 'x']	['var', 'x']	['var', 'x']	['var', 'x', '=']	['var', 'x', '=', '42']	['var', 'x', '=', '42', ';']	['var', 'x', '=', '42', ';']

Key Insights - 3 Insights

Why does the regex pattern for identifier not match the '=' symbol?

Because the identifier pattern only matches letters, digits, or underscores starting with a letter or underscore, '=' is not included, so no match occurs (see execution_table step 3).

What happens when no regex pattern matches the current input segment?

The lexer tries the next pattern or skips the character if none match, preventing infinite loops (see execution_table step 4).

How does the lexer know when to stop processing input?

When the input string is empty after extracting tokens, the lexer stops (see execution_table step 8).

Visual Quiz - 3 Questions

Look at the execution_table at step 2. What token is extracted from the input segment?

A number='x'

B identifier='x'

C symbol='x'

D No token extracted

💡 Hint

Check the 'Token Extracted' column at step 2 in the execution_table.

At which step does the lexer match the number token '42'?

A Step 6

B Step 5

C Step 4

D Step 7

💡 Hint

Look for the 'number' pattern match in the execution_table.

If the input started with '123var', which token would the lexer extract first?

A symbol='1'

B identifier='123var'

C number='123'

D No token extracted

💡 Hint

Recall that number pattern matches digits only and identifier must start with a letter or underscore.

Concept Snapshot

Regular expressions define patterns to identify tokens in input text.
Lexer applies these patterns sequentially to extract tokens.
Tokens include identifiers, numbers, symbols, etc.
If no pattern matches, lexer skips or errors.
Process repeats until input is fully tokenized.

Full Transcript

This visual execution trace shows how regular expressions are used to identify token patterns in a string. Starting with the full input, the lexer applies regex patterns like identifier and number to find matches. When a pattern matches, the corresponding token is extracted and stored, and the input moves forward. If no pattern matches, the lexer tries the next pattern or skips the character. This continues until the entire input is processed. The variable tracker shows how the input string shortens and tokens accumulate. Key moments clarify common confusions such as why certain characters don't match specific patterns and how the lexer knows when to stop. The quiz questions test understanding of token extraction steps and pattern matching order.