Regular expressions for token patterns in Compiler Design - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to match an identifier starting with a letter followed by letters or digits.

Compiler Design

pattern = r"[1]"

Drag options to blanks, or click blank then click option'

A"\d+"

B"[0-9]+"

C"[a-zA-Z][a-zA-Z0-9]*"

D"[a-z]+"

Attempts:

3 left

💡 Hint

Common Mistakes

Using digits at the start of the pattern

Not allowing digits after the first character

The pattern [a-zA-Z][a-zA-Z0-9]* matches an identifier that starts with a letter and is followed by zero or more letters or digits.

2fill in blank

medium

Complete the regular expression to match an integer literal consisting of one or more digits.

Compiler Design

pattern = r"[1]"

Drag options to blanks, or click blank then click option'

A"[a-zA-Z]+"

B"[0-9]*"

C"[a-z]+"

D"[0-9]+"

Attempts:

3 left

💡 Hint

Common Mistakes

Using * which allows zero digits

Using letters instead of digits

The pattern [0-9]+ matches one or more digits, which is suitable for integer literals.

3fill in blank

hard

Fix the error in the regular expression to correctly match a floating-point number with optional decimal part.

Compiler Design

pattern = r"\d+[1]\d*"

Drag options to blanks, or click blank then click option'

A"."

B"\."

C"\\."

Attempts:

3 left

💡 Hint

Common Mistakes

Not escaping the dot, causing it to match any character

Using too many backslashes

The dot . is a special character in regex that matches any character. To match a literal dot, it must be escaped as \..

4fill in blank

hard

Fill both blanks to create a regex that matches an identifier starting with a letter and containing letters, digits, or underscores.

Compiler Design

pattern = r"[1][2]*"

Drag options to blanks, or click blank then click option'

A"[a-zA-Z]"

B"[0-9]"

C"[a-zA-Z0-9_]"

D"[_]"

Attempts:

3 left

💡 Hint

Common Mistakes

Allowing digits or underscores as the first character

Not including underscore in the second part

The first blank matches the first letter [a-zA-Z]. The second blank matches zero or more letters, digits, or underscores [a-zA-Z0-9_]*.

5fill in blank

hard

Fill all three blanks to create a regex that matches a string literal enclosed in double quotes, allowing escaped quotes inside.

Compiler Design

pattern = r"\"[1][2]*[3]\""

Drag options to blanks, or click blank then click option'

A"(\\.|[^"]"

B"(\\.|[^"])*"

Attempts:

3 left

💡 Hint

Common Mistakes

Not escaping quotes inside the string

Not matching zero or more characters inside quotes

The pattern matches a double quote, then zero or more occurrences of either an escaped character (like \"), or any character except a double quote, and finally a closing double quote.

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