Question

Which of the following strings is NOT in the language described by the regular expression [a-zA-Z]([a-zA-Z$0-9]*)?

1. abc$21
2. a
3. $abc21
4. a21bc

Answer 1

The string '$abc2' is not in the language described by the regular expression [a-zA-Z]([a-zA-Z$0-9]*)

Step-by-step explanation:

The regular expression [a-zA-Z]([a-zA-Z$0-9]*) matches any string that starts with a single letter, followed by zero or more letters, numbers, or the dollar sign. So, let's analyze the given options:

1. abc$2: This string starts with a letter 'a' and is followed by two letters and a dollar sign, which matches the regular expression. So, this string is in the language described by the regular expression.
2. a: This string is just a single letter 'a', which also matches the regular expression.
3. $abc2: This string starts with a dollar sign, which does not match the requirement of the regular expression. Therefore, this string is NOT in the language described by the regular expression.
4. a21bc: This string starts with a letter 'a' and is followed by two numbers and two letters, which matches the regular expression. So, this string is in the language described by the regular expression.

Based on the analysis above, the string '$abc2' is NOT in the language described by the regular expression.