(m4.info)Index macro

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11.2 Searching for substrings
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Searching for substrings is done with 'index':

 -- Builtin: index (STRING, SUBSTRING)
     Expands to the index of the first occurrence of SUBSTRING in
     STRING.  The first character in STRING has index 0.  If SUBSTRING
     does not occur in STRING, 'index' expands to '-1'.

     The macro 'index' is recognized only with parameters.

     index(`gnus, gnats, and armadillos', `nat')
     =>7
     index(`gnus, gnats, and armadillos', `dag')
     =>-1

   Omitting SUBSTRING evokes a warning, but still produces output;
contrast this with an empty SUBSTRING.

     index(`abc')
     error->m4:stdin:1: Warning: too few arguments to builtin `index'
     =>0
     index(`abc', `')
     =>0
     index(`abc', `b')
     =>1