Question

1. regular expressions as a mechanism to specify tokens,

2. defining context-free grammars for language constructs, and

3. derivation of language terms and expressions based on a context-free grammar.

1 Regex

1. Define the regex for the following description of tokens:

(a) Any string that starts with character

t

(b) Any string of at least length 3 that starts with

t

and ends with

u

(c) Any string that specifies the range of numbers between 11 and 23.

(d) Any string that specifies a date in MM:DD:YYYY format.

2. In C, an identifier is defined as a string of characters (both upper-case and lower-case), digits, and

underscore “_”, starting with either a character or underscore. Define the regex for identifiers in C.

3. Give five strings that conform with the regex:

[0-9]+((E|e)(\+|\-)?[0-9]+)?

Answer 1

a) ^t - Regex for string starting with t (small letter t)

b) ^t.u - Regex for string starting from t and ending on u with 3 character

^ - Matches the beginning of the string, or the beginning of a line if the multiline flag (m) is enabled. This matches a position, not a character.

. - Matches any character except line breaks.

c) [1-2][1-3] ----- First digit in between 1-2 and second digit would be in between 1-3.

d) ^(1[0-2]|0[1-9]):(3[01]|[12][0-9]|0[1-9]):[0-9]{4}$ -----

Explanation: (1[0-2]|0[1-9]) - capturing group 1 which matches for month format , | works as OR condition

(3[01]|[12][0-9]|0[1-9]) - capturing group 2 for dd

[0-9]{4} - year validation

2) ^[_a-zA-Z][_a-zA-Z0-9]{0,30}

Explanation : ^ is used for start charater. {0,30} represnts upto 30 character long

3) strings matching regex:[0-9]+((E|e)(\+|\-)?[0-9]+)?

1e+9
4E-9
2E-5
9e+0
3e-2

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