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Substring ☆
1. A String variable can have a substring qualifier.
When s$ is a string variable, and m,n are numeric expressions, s$(m:n) stands for the substring of s$ from m-th character to n-th character.
For example, if a$="12345", a$(2:4) is "234".
2. A string variable with a substring qualifier can be written on the left hand side of a LET statement.
Example.
`LET s$(m:n)=a$`
The substring of s$ from m-th character to n-th character is replaced with a$.
The length of a$ can be different from the length of the substring to be replaced.
Example.

    10 LET A$="1234567"
    20 LET A$(2:6)="ABC"
    30 PRINT A$
    40 END

Output

    1ABC7

Insertion of a string to a string variable
`LET A$(n+1:n)=b$`
inserts b$ at the n-th character of A$.
Example.

    10 LET A$="1234567"
    20 LET LET A$(4:3)="ABC"
    30 PRINT A$
    40 END

Output 123ABC4567

Deletion of a substring from a string variable
`LET A$(m:n)=""`
deletes m-th character to n-th character from A$.
Example.

    10 LET A$="1234567"
    20 LET LET A$(4:6)=""
    30 PRINT A$
    40 END

Output 1237

(Note)
String variables with substring qualifiers can be written in a INPUT statement or a READ statement.
A substring qualifier can not be applied to a string expression.
A function that picks out a substring from a string can be defined as follows.
`DEF SUBSTR$(a$,m,n)=a$(m:n)`
SUBSTR$ is pre-defined on Decimal BASIC, so the DEF statement above need not be written.