Extracting Floating-Point Values from Strings with Regular Expressions

Consider the task of extracting a double value from a string. To achieve this using a regular expression, the following steps are involved:

1. Construct the Regexp:

   import re
   
   pattr = re.compile(???)
   x = pattr.match("4.5")

2. Use Perl-Compatible Regular Expressions:
   A suitable regexp from the Perl documentation for extracting floating-point values is:

   re_float = re.compile("""(?x)
   ^
      [ -]?\ *      # an optional sign and space
      (             # integers or f.p. mantissas
          \d        # start with a ...
          (           # ? takes care of integers
              \.\d* # mantissa a.b or a.
          )?
         |\.\d      # mantissa .b
      )
      ([eE][ -]?\d )?  # optionally match an exponent
   $""")

3. Find and Retrieve Matches:
   To extract the double value, apply the compiled regexp to the desired string:

   m = re_float.match("4.5")
   print(m.group(0))

   This will output:

   4.5

4. Extract Multiple Values from a String:
   To extract multiple floating-point values from a larger string, use the findall() method:

   s = """4.5 abc -4.5 abc - 4.5 abc   .1e10 abc . abc 1.01e-2 abc 
       1.01e-.2 abc 123 abc .123"""
   print(re.findall(r"[ -]? *(?:\d (?:\.\d*)?|\.\d )(?:[eE][ -]?\d )?", s))

   This will return a list of extracted values, including:

   ['4.5', '-4.5', '- 4.5', '  .1e10', ' 1.01e-2', '       1.01', '-.2', ' 123', ' .123']