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Editorial Team
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Asked: 2026-05-12T10:59:56+00:00

I am parsing documents which contain large amounts of formatted numbers, an example being:

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I am parsing documents which contain large amounts of formatted numbers, an example being:

 Frc consts  --     1.4362                 1.4362                 5.4100
 IR Inten    --     0.0000                 0.0000                 0.0000
 Atom AN      X      Y      Z        X      Y      Z        X      Y      Z
    1   6     0.00   0.00   0.00     0.00   0.00   0.00     0.00   0.00   0.00
    2   1     0.40  -0.20   0.23    -0.30  -0.18   0.36     0.06   0.42   0.26

These are separated lines all with a significant leading space and there may or may not be significant trailing whitespace). They consist of 72,72, 78, 78, and 78 characters. I can deduce the boundaries between fields. These are describable (using fortran format (nx = nspaces, an = n alphanum, in = integer in n columns, fm.n = float of m characters with n places after the decimal point) by:

 (1x,a14,1x,f10.4,13x,f10.4,13x,f10.4)
 (1x,a14,1x,f10.4,13x,f10.4,13x,f10.4)
 (1x,a4,a4,3(2x,3a7))
 (1x,2i4,3(2x,3f7.2))
 (1x,2i4,3(2x,3f7.2))

I have potentially several thousand different formats (which I can autogenerate or farm out) and am describing them by regular expressions describing the components. Thus if regf10_4 represents a regex for any string satisfying the f10.4 constraint I can create a regex of the form:

COMMENTS 
      (\s
      .{14}
      \s
      regf10_4,
      \s{13}
      regf10_4,
      \s{13}
      regf10_4,
)

I would like to know whether there are regexes that satisfy re-use in this way. There is a wide variety in the way computers and humans create numbers that are compatible with, say f10.4. I believe the following are all legal input and/or output for fortran (I do not require suffixes of the form f or d as in 12.4f) [the formatting in SO should be read as no leading spaces for the first, one for the second, etc.]

-1234.5678
 1234.5678
            // missing number
 12345678.
 1.
 1.0000000
    1.0000
        1.
 0.
        0.
     .1234
    -.1234
    1E2
    1.E2
    1.E02
  -1.0E-02
**********  // number over/underflow

They have to be robust against the content of the neighbouring fields (e.g. only examine precisely 10 characters in a precise position. Thus the following are legal for (a1,f5.2,a1):

a-1.23b   // -1.23
- 1.23.   // 1.23
3 1.23-   // 1.23

I am using Java so need regex constructs compatible with Java 1.6 (e.g. not perl extensions)

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1 Answer

  1. Editorial Team

    As I understand it, each line comprises one or more fixed-width fields, which may contain labels, spaces, or data of different kinds. If you know the widths and types of the fields, extracting their data is a simple matter of substring(), trim() and (optionally) Whatever.parseWhatever(). Regexes can’t make that job any easier–in fact, all they can do is make it a lot harder.

    Scanner doesn’t really help, either. True, it has predefined regexes for various value types, and it does the conversions for you, but it still needs to be told which type to look for each time, and it needs the fields to be separated by a delimiter it can recognize. Fixed-width data doesn’t require delimiters, by definition. You might be able to fake the delimiters by doing a lookahead for however many characters should be left in the line, but that’s just another way of making the job harder than it needs to be.

    It sounds like performance is going to be a major concern; even if you could make a regex solution work, it would probably be too slow. Not because regexes are inherently slow, but because of the contortions you would have to go through to make them fit the problem. I suggest you forget about regexes for this job.

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