you can insert spaces and linebreaks to be ignored when using the /x modifier.

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;
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