How do you convert a string containing a number in C scientific notation to a Mathematica number?

I think probably the cleanest way to do this (at least, if you have only a single string, or are faced with a separate string for each number you wish to convert as a result of some other process) is to use the undocumented function Internal`StringToDouble, i.e.:

s = "1.23e-5";
Internal`StringToDouble[s]

which gives:

0.0000123

However, if you are trying to convert many such numbers at once, the standard, documented methods (Import, Read, etc.), are likely to represent better approaches.

s = "1.23e-5"

# &[Read[#, Number], Close@#]&[ StringToStream@s ]

Which is not as good as what you started with. Note that it is important to close the stream.

---

Szabolcs says this is difficult to read. That was surely not my intention. You could also write it verbosely like this:

fromC =
    Module[{output, stream},
      stream = StringToStream[#];
      output = Read[stream, Number];
      Close[stream];
      output
    ] &;

fromC[s]

On version 7 Internal`StringToDouble fails on long strings, and fails to recognize exponents:

Internal`StringToDouble["3.1415926535897932385"]

Internal`StringToDouble /@ {"3.14159", "3.14159e-02", "3.14159e+02"}

$Failed["Bignum"]

{3.14159, 3.14159, 3.14159}

This sent me looking for another way to convert numeric strings. Using Trace on ImportString I found another internal function that does what I need: System`Convert`TableDump`ParseTable.

Being an internal function is it not error tolerant and if fed bad arguments it will crash the kernel. The syntax is as follows:

System`Convert`TableDump`ParseTable[
  table,
  {{pre, post}, {neg, pos}, dot},
  False
]

table  :   table of strings, depth = 2; need not be rectangular.  
pre    :   List of literal strings to ignore if preceding the digits (only first match tried).  
post   :   List of literal strings to ignore if following the digits (only first match tried).  
neg    :   literal string to interpret a negative sign (`-`).  
pos    :   literal string to interpret a positive sign (`+`).  
dot    :   literal string to interpret as decimal point.

(Using True in place of False causes a call to System`Convert`TableDump`TryDate that I do not yet understand.)

Example:

System`Convert`TableDump`ParseTable[
  {{"-£1,234.141592653589793e+007"}, {"0.97¢", "140e2kg"}},
  {{{"£"}, {"kg", "¢"}}, {"-", "+"}, "."},
  False
]

{{-1.2341415926535898*^10}, {0.97, 14000.}}