11.2 Searching for substrings ¶

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