Question

I once saw a regular expression example

^\s*(+-)?(\d+|\d*\.\d+|\d+\.\d*)(e?(+-)?......$

but I just do not know how to understand/analyze this one?

Answer 1

Analyze it a bit at a time; if you see a ?, *, or + then that modfies the previous ‘group’ (parenthesized group, or ‘atomic’ unit such as \s or a literal like e):

^            The start of the string
\s*          Zero or more whitespace characters.
(+-)?        An optional + or -
(
   \d+       One or more digits
 |           or
   \d*\.     Zero or more digits, followed by a .
   \d+       One or more digits
 |           or
   \d+\.     At least one digit, followed by a .
   \d*       Zero or more digits
)
(
 e?          An optional e
 (+-)?       An optional + or -

Here the regex trails off, and I’m not sure if the ..... is a literal series of .s in the regex or just a normal ellipsis. But hopefully this is a clear explanation. (by the way, $ means ‘end of the string’).