How to Extract Double Precision Floating-Point Values from Strings with Regex?

Extracting Double Precision Floating-Point Values from Strings with Regular Expressions

In this article, we will explore a question often encountered in programming: how to extract double precision floating-point numbers from a text string using the Python re module for regular expressions.

Regex Pattern for Double Precision

To match double precision floating-point values, we can use a regular expression that captures optional signs, integral or fractional parts, and an optional exponent. The following pattern is an example from the Perl documentation:

<code class="python">re_float = re.compile("""(?x)
   ^
      [+-]?\ *      # optional sign and space
      (             # integer or fractional mantissa:
          \d+       # start out with digits...
          (
              \.\d* # mantissa of the form a.b or a.
          )?        # ? for integers of the form a
         |\.\d+     # mantissa of the form .b
      )
      ([eE][+-]?\d+)?  # optional exponent
   $""")</code>

Matching and Extracting

To match a double precision value with this pattern, we can use the match method on a compiled regular expression object:

<code class="python">m = re_float.match("4.5")
print(m.group(0))  # prints 4.5</code>

This extracts the portion of the string that matches the pattern, in this case, the entire string.

Extracting Multiple Values

If we have a larger string containing multiple double precision values, we can use the findall method to extract all matching values:

<code class="python">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))
# prints ['4.5', '-4.5', '- 4.5', '+ .1e10', ' 1.01e-2',
#     '       1.01', '-.2', ' 123', ' .123']</code>

This pattern matches any double precision floating-point value, regardless of spaces or surrounding text, and extracts it as a list of strings.

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