CODEWITHSUNDEEP
regexpdfghostscript
Ooker
Ooker

Reputation: 3044

Is it possible to make an index search by regex in PDF?

I want to search for all lines that match this regex

^([0-9IVX]\.)*.*\R

and report with the page number they are at. The output would be something like:

1. Heading/page number
1.1 Subheading/page number
1.1.1. Subsubheading/page number

Is this possible to do in PDF? I suppose that would require Ghostscript, but searching the How to Use Ghostscript page for regex I find nothing.

Upvotes: 0

Views: 384

Answers (1)

KenS
KenS

Reputation: 31199

I can't think why you would expect Ghostscript to do search for you.

I'm not sure if you are hoping to get the data type 'heading, page number' etc from the PDF file, or if you are going to work that out yourself based on the data you find.

If it's the former then the first problem is that, in general, PDF files don't have the kind of structure information you are looking for. There is nothing in most PDF files which says 'this is a heading', 'this is a page number' etc.

There are such things as 'tagged PDF' which adds non-printing elements to a PDF file which do carry that kind of data around with them. This is an entirely optional feature, the vast majority of PDF files don't contain it, and Ghostscript completely ignores it.

Since most PDF files don't have that information, you can't rely on it, unless you are in the happy position of knowing where your PDF files are being generated and that they contain this kind of information. In which case there are numerous tools around which will extract it for you, or enable you to write code to do so.

The problem with just searching for the text is that firstly the text need not be written as a contiguous stream. So if you are looking for '1.1' that might be written as:

(1.1) Tj

(1) Tj
(.) Tj
(1) Tj

[(1) -0.1 (.) 0.1 (1)] TJ

or any combination of those. The individual character codes need not even appear in order or in the same content stream.

Secondly the character code in a PDF content stream need not be (and often is not) a Unicode code point. Or ASCII, or any other standard coding scheme, it can be totally arbitrary.

Some PDF files carry a ToUnicode CMap around which maps the character codes to Unicode code points, but not all do. Some fonts may use a standard (that's PDF standard) Encoding, in which case it's possible to infer the Unicode code points. Some Encodings may contain glyph names, from which it's again possible to infer Unicode code points.

In the end though, some PDF files are simply impossible to extract text from without using OCR.

Your best bet is probably to write code to extract text, and Ghostscript will do that. It even goes through the heirarchy of fallbacks listed above to try and find a Unicode code point. If all else fails it just uses the character code and hopes that's good enough.

If you use Ghostscript's txtwrite device it will produce either a faked up text page (the default) which attempts, as far as possible, to mimic the text layout in the original PDF file, including merging bits of text that aren't contiguous in the PDF file but are next to each other on the page. Or an 'XML-like' output which will tell you which Unicode code points, or character codes, were encountered and what their position is on the original page. If you don't like txtwrite's attempts to figure out which text goes with what, then you can use this to write your own.

I suspect the text page is probably good enough for your purposes. You can have the txtwrite device produce one file per page, so you can get the page number from the filename. Then you can write your own regex expression(s) to search the files and find your matches.

Upvotes: 1

Related Questions