Question

Which of the following strings is not in the language described by the regular expression [a-za-z]([a-za-z$0-9]*)?

a. abc$21

b. a

c. $abc21

d. a21bc

Answer 1

The string that is not in the language described by the regular expression is $abc21 because it starts with a dollar sign, and the regular expression requires the first character to be an alphabetical character.

Step-by-step explanation:

The student's question asks which string among the given options is not in the language described by the regular expression [a-za-z]([a-za-z$0-9]*). This regular expression can be broken down as follows: it starts with a lowercase or uppercase alphabetical character [a-za-z], followed by zero or more characters that can be lowercase or uppercase alphabets, dollar signs, or digits [a-za-z$0-9]*.

Looking at the options:

• abc$21 - Starts with an alphabet and contains valid characters according to the regular expression.
• ac - Both characters are alphabets, so this string is valid.
• $abc21 - This string is not valid because it starts with a dollar sign, which is not allowed by the regular expression's first character requirement.
• a21bc - Starts with an alphabet and follows with valid characters, making it valid.

Therefore, the string that is not in the language described by the regular expression is $abc21.

Answer 2

The string that is not in the language described by the regular expression [a-za-z]([a-za-z$0-9]*) is c. $abc21.

Step-by-step explanation:

The regular expression [a-za-z]([a-za-z$0-9]*) specifies a pattern for strings in which the first character is a lowercase or uppercase letter, and the subsequent characters can be a combination of letters, digits, or the dollar sign ($). Let's analyze each option:

a. abc$21 - This string is in the language as it starts with a letter 'a' and is followed by a combination of letters, digits, and the dollar sign.

b. a - This string is in the language as it consists of a single letter 'a'.

c. $abc21 - This string is not in the language as it starts with a non-alphabetic character ('$'), violating the regular expression's requirement for the first character to be a letter.

d. a21bc - This string is in the language as it starts with a letter 'a' and is followed by a combination of letters and digits.

In summary, option c. $abc21 is the string that does not conform to the specified language defined by the given regular expression. It violates the requirement for the first character to be a letter. The other options align with the defined pattern, making them part of the language described by the regular expression.