Question

Let digit be [0-9] what does the following regular expression defines in Unix?
a). [-]?(({digit}+)|({digit}*\.{digit}+)([eE][+-]?{digit}+)?)
b). [-+]?{digit}+
c). Give two examples for each of the above.

Answer 1

a) [-]?(({digit}+)|({digit}*\.{digit}+)([eE][+-]?{digit}+)?)
Let me explain in parts -
[-]? -> 0 or 1 occurrence of '-'
({digit}+)|({digit}*\.{digit}+) ->  {digit}+ -> 1 or more occurrence of digits
| -> OR
  {digit}*\.{digit}+ -> 0 or more occurrence of digits followed by a '.' and one or more occurrence of digits
([eE][+-]?{digit}+)? -> [eE][+-]? -> 'e' or 'E' followed by either (+ or -)(0 or 1 occurrence)
{digit}+ -> 1 or more occurrence of digits
? -> 0 or 1 occurrence of ([eE][+-]?{digit}+)
Example - 1, .9, 0.2, 7e9, 7e-9, 7e+9, 0.2E+9
Basically, it is representation in exponential form with the e part may or may not exist

b) [-+]?{digit}+
It is simply representation of integers that is (+ or -)(0 or 1 occurrence) followed by at least 1 digit.
Example - 1,+1,-1