GNU Info

(m4.info)Index

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

     index(STRING, SUBSTRING)

which 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'.

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

   The builtin macro `index' is recognized only when given arguments.