What are extended regular expressions?

Question

I am studying for an exam and one of the topics is on RegEx. There is a modifier, x, with the description 'uses extended regular expressions,' but it doesn't explain what this means. I can't really find anything on Google that fits the context of my study material.

On a quiz, one of the questions asked what modifier allows the use of comments in RegEx, and the correct(?) answer was x.

Could someone please help me make sense of this? Thank you.

Edit: I meant x in the context of [gix], where x is described as 'uses extended regular expressions.'

Answer 1

As documented in perlretut (Just search for /x):

Long regexps like this may impress your friends, but can be hard to decipher. In complex situations like this, the //x modifier for a match is invaluable. It allows one to put nearly arbitrary whitespace and comments into a regexp without affecting their meaning. Using it, we can rewrite our 'extended' regexp in the more pleasing form

    /^
          [+-]?         # first, match an optional sign
          (             # then match integers or f.p. mantissas:
               \d+\.\d+  # mantissa of the form a.b
              |\d+\.     # mantissa of the form a.
              |\.\d+     # mantissa of the form .b
              |\d+       # integer of the form a
          )
          ([eE][+-]?\d+)?  # finally, optionally match an exponent
      $/x;

Answer 2

As stated in the perlre documentation:

/x tells the regular expression parser to ignore most whitespace that is neither backslashed nor within a character class. You can use this to break up your regular expression into (slightly) more readable parts...

The modifier improves readability, and allows us to add explanatory comments.

 /^            # the beginning of the string
   (?:         # group, but do not capture:
       foo     #   match 'foo'
      |        #  OR
       bar     #   match 'bar'
   )           # end of grouping
  $            # the end of the string
 /x;

Even without the /x modifier you can enclose comments.

/foo(?# match foo)/

Answer 3

/x to ignore whitespace that is neither backslashed nor within a character class. to make your regex more readable