m/
  ^          # anchor to the beginning of the string
  -?         # 0 or 1 dashes (for negative numbers I assume)
  \.?        # 0 or 1 decimal points
  \d+        # 1 or more numbers
  (?:\.\d+)? # 0 or 1 decimal places followed by a number
  $          # anchor to the end of the string
/x
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So basically what you have is a a possibly negative, possibly decimal number (-?\.\d+), followed by an optional decimal and number.

# some examples
/^-?\.?\d+/ matches 
    123 or 
    -123 or
    -.123 or
    .123
/^-?\.?\d+(?:\.\d_)?/ matches those and also 
    123.45 or 
    -123.45 or
    -.123.45 (probably a bug!) or
    -123.23532 or
    .235235235235.235235235235235325
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The (?:...) construct lets you use parentheses for grouping, without assigning them to the $n variables, so your regexp contains parens that keep the \. and \d+ together, but doesn't assign the match to $1.