Regular expressions for token patterns in Compiler Design - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a regular expression in the context of token patterns?

A regular expression is a sequence of characters that defines a search pattern used to identify tokens in text, such as keywords, identifiers, or symbols in programming languages.

beginner

How do regular expressions help in lexical analysis?

They provide a formal way to describe the patterns of tokens, allowing the lexer to recognize and extract meaningful units from the source code efficiently.

intermediate

What does the regular expression [a-zA-Z_][a-zA-Z0-9_]* represent?

It represents an identifier token pattern: it starts with a letter or underscore, followed by zero or more letters, digits, or underscores.

beginner

Explain the difference between * and + in regular expressions.

* means zero or more repetitions of the preceding element, while + means one or more repetitions.

intermediate

Why are regular expressions not suitable for parsing nested structures?

Because regular expressions cannot handle recursive or nested patterns, which require more powerful tools like context-free grammars.

Which regular expression matches a decimal number with optional fractional part?

A \d+(\.\d+)?

B [a-zA-Z]+

C \w+

D \d*

Option A matches one or more digits followed optionally by a dot and one or more digits, representing a decimal number.

What does the regular expression if|else|while represent?

A Any identifier

B Keywords 'if', 'else', or 'while'

C Numbers

D Whitespace characters

The expression matches exactly the keywords 'if', 'else', or 'while'.

In regular expressions, what does the symbol \w typically represent?

A Any whitespace character

B Any digit

C Any special character

D Any word character (letters, digits, underscore)

\w matches letters, digits, and underscore characters.

Which of the following is NOT a valid use of regular expressions in token patterns?

A Matching nested parentheses

B Matching identifiers

C Matching numeric literals

D Matching keywords

Regular expressions cannot match nested structures like parentheses because they lack memory for recursion.

What does the regular expression [0-9]+ match?

A Any letter

B Zero or more digits

C One or more digits

D Whitespace

The expression matches sequences of one or more digits.

Describe how regular expressions are used to define token patterns in lexical analysis.

Think about how a lexer uses patterns to find meaningful pieces in code.

Explain the limitations of regular expressions when used for parsing programming languages.

Consider what kinds of language features are too complex for regex.

Practice

1. What is the main purpose of using regular expressions in compiler design?

easy

A. To define patterns for recognizing tokens in source code

B. To execute arithmetic operations

C. To optimize machine code

D. To manage memory allocation

Solution

1. Step 1: Understand the role of regular expressions

   Regular expressions are used to describe patterns that match sequences of characters.

2. Step 2: Connect to compiler design

   In compiler design, these patterns help identify tokens like keywords, identifiers, and symbols.

3. Final Answer:

   To define patterns for recognizing tokens in source code -> Option A

4. Quick Check:

   Regular expressions = token patterns [OK]

Hint: Regular expressions match text patterns for tokens [OK]

Common Mistakes:

• Confusing regular expressions with code execution
• Thinking they optimize code performance
• Mixing up with memory management

2. Which of the following is the correct regular expression to match an identifier starting with a letter followed by letters or digits?

easy

A. [a-zA-Z][a-zA-Z0-9]*

B. [0-9][a-zA-Z]*

C. [a-zA-Z0-9]+[a-zA-Z]*

D. [0-9]+[a-zA-Z0-9]*

Solution

1. Step 1: Identify the pattern for an identifier

   An identifier starts with a letter, then can have letters or digits.

2. Step 2: Match the regular expression

   [a-zA-Z][a-zA-Z0-9]* matches one letter [a-zA-Z] followed by zero or more letters or digits [a-zA-Z0-9]*.

3. Final Answer:

   [a-zA-Z][a-zA-Z0-9]* -> Option A

4. Quick Check:

   Letter + letters/digits = [a-zA-Z][a-zA-Z0-9]* [OK]

Hint: Start with letter, then letters or digits [OK]

Common Mistakes:

• Starting identifier with a digit
• Using + instead of * incorrectly
• Allowing digits as first character

3. Given the regular expression \d{3}-\d{2}-\d{4}, what kind of token does it match?

medium

A. A date in format YYYY-MM-DD

B. A phone number with area code

C. A US Social Security Number format

D. An IP address

Solution

1. Step 1: Analyze the regular expression pattern

   \d{3} means exactly 3 digits, then a dash, then 2 digits, dash, then 4 digits.

2. Step 2: Identify the token format

   This matches the US Social Security Number format: 3 digits - 2 digits - 4 digits.

3. Final Answer:

   A US Social Security Number format -> Option C

4. Quick Check:

   \d{3}-\d{2}-\d{4} = SSN format [OK]

Hint: Count digits and dashes to identify pattern [OK]

Common Mistakes:

• Confusing with date formats
• Assuming phone number format
• Ignoring exact digit counts

4. Identify the error in this regular expression intended to match floating-point numbers: \d+\.\d*

medium

A. It requires at least one digit after decimal point

B. The dot is not escaped properly

C. It matches only integers

D. It does not allow numbers without decimal part

Solution

1. Step 1: Understand the pattern meaning

   \d+ means one or more digits, \. means a literal dot, \d* means zero or more digits after dot.

2. Step 2: Check what numbers it matches

   This matches numbers with a decimal point and optional digits after it, but does not match integers without a dot.

3. Final Answer:

   It does not allow numbers without decimal part -> Option D

4. Quick Check:

   Pattern requires dot, so no integers [OK]

Hint: Check if integers without dot are matched [OK]

Common Mistakes:

• Thinking dot is unescaped
• Assuming digits after dot are mandatory
• Confusing * and + quantifiers

5. How would you write a regular expression to match either the keyword if or else but not identifiers containing these as substrings?

hard

A. (if|else)+

B. \b(if|else)\b

C. if|else

D. [ifelse]+

Solution

1. Step 1: Understand the requirement

   We want to match exactly the words 'if' or 'else', not parts of longer words.

2. Step 2: Use word boundaries

   \b marks word boundaries, so \b(if|else)\b matches only whole words 'if' or 'else'.

3. Step 3: Check other options

   (if|else)+ allows repeated keywords, if|else matches substrings anywhere, [ifelse]+ matches any combination of letters i,f,e,l,s.

4. Final Answer:

   \b(if|else)\b -> Option B

5. Quick Check:

   Word boundaries ensure exact keyword match [OK]

Hint: Use \b for exact word matching [OK]

Common Mistakes:

• Not using word boundaries
• Using character classes instead of groups
• Allowing partial matches inside identifiers