It is often helpful to be able to display calculated values in an HTML file. And when using the .IF command, you will often want to have it act differently based on some logical condition.

The Document Interface supports a number of arithmetic and logic operators, all of which are actually handled by the Perl language. The operators themselves must not be within either type of quotes, even if the operands are.

Operations are normally evaluated using set rules of precedence. This means that, for instance, multiplication is carried out before addition. To override this, operations can be surrounded by any number of parentheses () to give them precedence. For example, the following operations produce the results on the right:

Operation	Result	Reason
2+3*4 (2+3)*4 2==2 || 1==2 && 3==3 || 3==4 (2==2 || 1==2) && (3==3 || 3==4)	14 20 false true	multiplication * evaluated first logical AND && evaluated first

The following table lists the operators in order of precedence, with the highest precedence listed at the top. Operations of equal precedence are always evaluated from left to right.

Numeric, String, and Logic Operators

Operator	Description	Example	Result
#string#expression#	Returns the portion of string which matches the Perl regular expression, or an empty string if there is no match. For a full description of this advanced feature, see the Perl page on regular expressions.	#abcdef#a[bc]# #abcdef#a.*e#	ab abcde
**	Exponentiation	10**2 4**3	100 64
!	Logical negation (Not)	!(1==2) !1 || !0	true true
* / %	Multiplication Division Modulo	10*3 + 24/6 10*(3 + 24)/6 11/4 11%4	34 45 2.75 3
+ - .	Addition Subtraction String concatenation	3+4 3-4 "abc"."ab"."z"	7 -1 abcabz
< > <= >= lt gt le ge	Less than Greater than Less than or equal to Greater than or equal to Alphabetically less than Alphabetically greater than Alphabetically less than or equal to Alphabetically greater than or equal to	1<2 1>2 1+2<=3 1+2>=4 "abc" lt "def" "abc" ge "a" "abc" le "abc" "aBc" ge "abc"	true false true false true true true false
== != eq ne	Equals Does not equal String equals String does not equal	9*5==5 10/3!=3 "abc" eq "Abc" "abc" ne "abcd"	false true false true
&&	Logical And	1+2==3 && 2>1	true
||	Logical Or	1+2==3 || 1>2 2==0 && 1==1 || 2==2	true true

All of the string operators (lt, ge, ne, etc.) are case-sensitive. You have to enter them in lowercase.

When using logical expressions with the .IF command, you should keep in mind that the code contained within the .IF will not be evaluated if the argument passed to it is either the number 0, an empty string, or any of the strings "no", "off", or "false" (case does not matter.) This means that all of the following examples produce the same result:

<!--.if 0-->
<!--.if fALse-->
<!--.if 1==99 || 4+4==9-->

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