FIND

The function FIND searches for a partial string within a string.
FIND (STR1, STR2) means: Find the position of the first character where STR2 appears in STR1 for the first time. If STR2 is not found in STR1, then OUT:=0.

VAR_INPUT
    STR1 : STRING(255);
    STR2 : STRING(255);
END_VAR

LD 'SUXYSI'
FIND 'XY'
ST Var1 (* Result is 3 *)

Var1 := FIND('SUXYSI','XY');