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BASIC has classic string functions, mostly with $ suffix in older dialects (FreeBASIC accepts both).

DIM s AS STRING = "hello world"

LEN(s)               ' 11
UCASE(s)             ' HELLO WORLD
LCASE("HI")          ' hi

MID(s, 7, 5)         ' "world" — substring (start, length)
LEFT(s, 5)           ' "hello"
RIGHT(s, 5)          ' "world"

INSTR(s, "world")    ' 7 — position (1-indexed) or 0 if not found
INSTR(s, "xyz")      ' 0

STR(42)              ' " 42" (note leading space for sign)
VAL("42")            ' 42 — parse to number

LTRIM("  hi")        ' "hi" — remove leading whitespace
RTRIM("hi  ")        ' "hi"
TRIM("  hi  ")       ' "hi"

Concatenation — both + and & work:

DIM greeting AS STRING = "Hello, " + name + "!"

Replacing isn't built into all BASIC dialects. FreeBASIC has Replace:

#include "string.bi"   ' if needed
' or just write your own using INSTR + LEFT + RIGHT

Split / parse — older BASIC has no built-in split. Common pattern:

Function CountWords(s AS STRING) AS INTEGER
    DIM count AS INTEGER = 0
    DIM i AS INTEGER
    DIM in_word AS INTEGER = 0
    FOR i = 1 TO LEN(s)
        IF MID(s, i, 1) = " " THEN
            in_word = 0
        ELSE
            IF in_word = 0 THEN count += 1
            in_word = 1
        END IF
    NEXT i
    Return count
End Function