In short: the intent is it matches numbers that look like -1.2, 2.347, 47., -.58 and similar. Flag /g means global, that is to say match several times on the target variable $line, not only once. Read any book or tutorial on regex. There are also many Web sites that analyse and explain regex similar to the output below. 
YAPE-Regex-Explain-4.01› perl examples/explain
\-?(?:\d+\.?\d*|\.\d+)
The regular expression:

(?-imsx:\-?(?:\d+\.?\d*|\.\d+))

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  \-?                      '-' (optional (matching the most amount
                           possible))
----------------------------------------------------------------------
  (?:                      group, but do not capture:
----------------------------------------------------------------------
    \d+                      digits (0-9) (1 or more times (matching
                             the most amount possible))
----------------------------------------------------------------------
    \.?                      '.' (optional (matching the most amount
                             possible))
----------------------------------------------------------------------
    \d*                      digits (0-9) (0 or more times (matching
                             the most amount possible))
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    \.                       '.'
----------------------------------------------------------------------
    \d+                      digits (0-9) (1 or more times (matching
                             the most amount possible))
----------------------------------------------------------------------
  )                        end of grouping
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

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In reply to Re: Print the line with the largest number from standard input by daxim
in thread Print the line with the largest number from standard input by SSSufe

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