Use character substitution in BibTeX .bst file

Question

I use BibTeX not only to create bbl files for my LaTeX documents, but also to produce HTML of my whole collection of articles. It works pretty well for the last 12 years but since I decided to add a DOI field I've got a minor problem.

BibTeX, outputs

(<small>doi:<a href=http://dx.doi.org/10.1051/jp2:1995145>10.1051/jp2:1995145</a></small>)

as I programmed it like

FUNCTION {startdoienc}
{ duplicate$ empty$
    { pop$ "" }
    { " (<small>doi:<a href=http://dx.doi.org/" swap$ * ">" * }
  if$
}

FUNCTION {enddoienc}
{ duplicate$ empty$
    { pop$ "" }
    { "</a></small>)" * }
  if$
}

FUNCTION {article}
{ output.bibitem
  .... <cut out>
  new.block
  doi startdoienc  output
  doi enddoienc  pureoutput 
  new.block
  .... <cut out>
}

So the HTML result looks like

A. Ajdari, F. Brochard-Wyart, C. Gay, P. G. de Gennes, and J. L. Viovy Drag on a tethered chain moving in a polymer melt J. Phys. II France 1995 5 491–495 (doi:10.1051/jp2:1995145)

where the DOI link is clickable.

However, some journals (e.g. J. Polym. Sci. B: Polym. Phys.) have characters in their DOI's, which need to be escaped to become a valid URL: <, >, and #. Example:

10.1002/(SICI)1099-0488(19990701)37:13<1449::AID-POLB11>3.0.CO;2-T

Question: is it possible to program BibTeX to substitute, e.g., "<" by "%3c" before outputting it?

Answer 1

Search-and-replace is possible using BibTeX. The excellent Tame the BeaST suggests

INTEGERS{ l }
FUNCTION{ string.length }
{
  #1 'l :=
  {duplicate$ duplicate$ #1 l substring$ = not}
    {l #1 + 'l :=}
  while$
  pop$ l
}

STRINGS{replace find text}
INTEGERS{find_length}
FUNCTION{find.replace}
{ 'replace :=
  'find :=
  'text :=
  find string.length 'find_length :=
  ""
    { text empty$ not }
    { text #1 find_length substring$ find =
        {
          replace *
          text #1 find_length + global.max$ substring$ 'text :=
        }
        { text #1 #1 substring$ *
          text #2 global.max$ substring$ 'text :=
        }
      if$
    }
  while$
}

So you should be able to do something like

FUNCTION {startdoienc}
{ duplicate$ empty$
    { pop$ "" }
    {
       " (<small>doi:<a href=http://dx.doi.org/" swap$ 
       "<" "\%3c" find.replace 
       * ">" *
    }
  if$
}

You may also need to escape the % to stop it being read as a comment by LaTeX.