how to use variables in sql

I'm beginner in Oracle. I'm wondering how to use variables in SQL, to make statement less complicated. For example:

SELECT      substr(nnp,instr(nnp, '*')+1, instr(nnp, '/')+1 - instr(nnp, '*')+1)FROM dav_5;

i want to make like this:

A := instr(nnp, '*')+1
B := instr(nnp, '/')+1
FROM dav_5

What should I do, to run this in SQL Plus?
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define A = "instr(nnp, '*')+1"
define B = "instr(nnp, '/')+1"
SELECT SUBSTR (nnp, &A, &B-&A)
FROM dav_5

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As your variable containts single quotes, you need to define them with enclosing double quotes

You need to use Define to declare variables which can be used in SQL.

you just give the comman DEFINE to see what all variable are already declared.

SQL> ACCEPT A CHAR PROMPT 'Enter Value of First Expression'
SQL> ACCEPT B CHAR PROMPT 'Enter value of second Expression'
SQL> SELECT SUBSTR (nnp, &A, &B-&A) FROM dav_5

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