File: m4.info,  Node: Index macro,  Next: Regexp,  Prev: Len,  Up: Text handling

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