The macro \doifnumberelse is not behaving as I would have expected from the description in Taco Hoekwater's article "Context System Macros: part 1: General macros".

There I read:

  The next macro executesa command depending of the outcome of a test on numerals.

  \doifnumberelse {string} {then ...} {else ...}

This text suggest that the the test is on whether "string" as a whole is a number (taken as having digits 0123456789 only, I presume) of contains nondigits and thus isn't a number.

However, it seems the macro tests on the first character only.

Below follows a minimal example, the context version is:

% test result of \doifnumberelse

\def\test#1{#1 is \doifnumberelse{#1}{a number}{not a number}\crlf}

\starttext

\test{1}   % result: 1 is a number

\test{a}   % result: a is not a number

\test{1a}  % result: 1a is a number

\test{a1}  % result: a1 is not a number

\stoptext