Current Version: 1.0.29
Project Name: csspp
CSS Preprocessor Reference -- Expressions by Type

Table of Contents

The CSS Preprocessor supports many operations of the many types it supports. The following entries describe the various capabilities offered by the expression part of the compiler.


Some identifiers are viewed as the named representation of a value. The following are the values that get converted when an identifier is found as a unary value.

The other use is a color name. In that case, the identifier is transformed in a color object. The assembler output capability will transform colors to the smallest possible representation which may be an identifier (i.e. the color red is generally output as the identifier red.)

When appearing in place of an operator, the identifier may be converted to such. A certain number of operators were given common names as found in other languages and in SASS. There are the operators we currently support as identifiers (operator, identifier, full name):

The pow keyword only represents the power operator. We do not offer a function since one can use the operator.


Strings can be concatenated using the + operator. The operation is not commutative.

"Hello " + "world" + "!" = "Hello world!"

Strings can be duplicated using the * operator. The operation is commutative.

"Ho! " * 3 = "Ho! Ho! Ho! "

Unicode Ranges

The -face keyword accepts a field named unicode-range which accepts ranges of Unicode character code points.

These ranges can be intersected between each others uing * operator. The operation is commutative.

U+1??? * U+17?? = U+17??

As a side effect, the union accepts NULL as one or both input. The resulting range will always be NULL.

U+1??? * U+7?? = NULL
// and thus
U+1??? * U+7?? * U0???-1??? = NULL

At this time we do not allow for a union because we cannot give a valid answer if the intersection of two ranges is null when dealing with a union (i.e. a unicode range with a gap requires two ranges.)


The two Boolean values are true and false. The relation operators return Boolean values. Boolean values can be used with the || and the && operators. They are also used as the first parameter of the conditional operation (<boolean> ? <value if true> : <value if false>).

Note that the system also views all numbers as boolean values when used in a context where a Boolean value is expected in that case a number which is equal to zero represents false. Any other number represents true.

Similarly, strings that are empty represent false and strings that are not empty represent true. Note that the '\0' character does not make it in our strings so it can be used as a string terminator and thus a string with just '\0' is considered empty.

Maps and arrays can also be converted to a Boolean value. An empty map or array represent the value false. A non empty map or array represent the value true.

The NULL token is viewed as false.

The black color, whatever the alpha level (red = 0.0, green = 0.0, and blue = 0.0), is considered false. Any other color is considered true.

An identifier other than null, true, and false is considered to be an invalid Boolean value. Note that in expression, identifiers that represent a color will be transformed in that color first and thus the color scheme applies (i.e. black is false, chocolate is true.)

Numbers (integers and decimal numbers)

All operators accept numbers (integers and decimal numbers.)

Operators that expect booleans (&& and ||) transform numbers to true unless they represent zero.

Percent Operations

Percent numbers can be compared, added, subtracted between each others.

Percent numbers can be used to multiply and divide any other number, including numbers with dimensions and other percentages. The results are just as expected: a multiplication or division of that number by a unitless number equal to the percent number divided by 100. In this case, the other number dimension does not change.

13px * 50% = 7.5px

Note that percent numbers are always considered to be decimal numbers and therefore the result is always a decimal number even if the other number was an integer and if the precent number looks like an integer too.

A percent number at any given power remains a percent number.

Numbers and Relations

These operators always return true or false. You may compare integers against decimal numbers. The compiler does not currently issue a warning if you use the = and != operators against decimal numbers.

Arithmetic Operations

When dealing with integers, the results are always integers:

\[ { 10 \over 3 } = 3 \tag{Integer} \]

Expressions that involve at least one decimal number always return a decimal number:

\[ { 10 \over 3.0 } = 3.\overline{3} \\ or \\ { 10.0 \over 3 } = 3.\overline{3} \\ or \\ { 10.0 \over 3.0 } = 3.\overline{3} \tag{Decimal Number} \]

The modulo operator (%) can be used with floating point numbers (see man fmod() for details.)

When expressions are executed with a decimal number, then results are always decimal numbers. However, if the final decimal number can be represented as an integer, the assembler output will be an integer. In other word, there is a loss of information for the convenience of the final minified CSS output.

Arithmetic Operations on Dimensions

Just like with unitless numbers, you may apply arithmetic operations against numbers that have dimenions.

Relations require the left and right hand side numbers to all have the same dimensions:

\[ 3px < 7px \tag{Correct} \]

whereas the following generates an error:

\[ 3px < 7em \tag{Wrong} \]

Additions (+), subtractions (-), and modulo (% or mod) are like relations, they only accept the exact same dimensions on the left and right hand side to function.

Multiplication and Dimensions

Multiplications work with any dimensions as follow. The result is a set of mixed dimensions:

\[ 3px \times 7em = 21px \cdot em \tag{Multiplication} \]

In CSS Preprocessor files (.scss files) you may enter such dimensions using the asterisk as in (the backslash is required):


The power operator works in a similar way to the multiplication operator. The power has to be a unitless number and must be a positive integer ( \(power \in \Bbb{Z}^*\)). In all other cases the power operator generates an error if the left hand side number has a dimension. The result looks like this:

\[ (3px) ^ 5 = 3px^5 \\ since \\ 3px \times 3px \times 3px \times 3px \times 3px = 3px^5 \tag{Power} \]

Division and Dimensions

Divisions work with any dimensions. The result is a set of mixed dimensions.

\[ { 21px \over 7em } = 3px/em \\ or \\ { 21px \over 7em } = 3px \cdot em^{-1} \tag{Division} \]

When writing such in CSS Preprocessor files, dimensions found on the right side of the '/' operator are put after a slash in the list of dimensions (the backslash is required):


When you have more than one dimension over or under, all dimensions that are over are written before the slash (/), and all dimensions that are under are written after the slash. All dimensions over or under are separated by an asterisk (*).


Note that you should not write the same dimension before and after the slash (/) since that represents 1, unitless.

Internally, dimensions make use of a space (one of my previous example would look like this string, internally: "px * em * cm / vw * mm".) We do not force you to add spaces. However, you can only put 0 or 1 space at this time. I may fix that problem later (i.e. allow any number of spaces.) Spaces are supported for SASS compatibility. A space at the end of a dimension is also accepted.

Only Inverted Dimensions

When dealing with numbers, it may happen that you end up with an inverted dimension only. In math or physics, we pretty much never write such as a result, but if you were to do so, it would look like this:

\[ { 1 \over 3 \, m } = 3 m^{-1} \tag{Inverted} \]

For CSS Preprocessor to support any type of units while operating on various numbers, this case happens, so it had to have a way to allow such inverted dimensions. This is done by using "1 /" as an introducer. So the previous example could be written as follow in an .scss file (backslashes are required):


Note that the dimension has to start with a 1 which is why you need to write that digit using the backslash syntax and '31' represents the ASCII code in hexadecimal.

Dimensions Simplifications

When multiplying or dividing two numbers against each others, the system computes a new set of dimensions before the slash (/) and a list of dimensions after the slash (/). If any dimension is found in both lists, then they both get removed. As a result, a dimension can only be found at the first or the second list, not both. However, it can be duplicated in either list (i.e. \(cm^2\) is represented as "cm * cm").

These operations allow you to remove dimensions from a number. For example, to transform a size in pixels to a unitless number, you may do the following (see also remove_unit(), unit(), and unitless()):

15px / 1px = 15

Similarly, you can convert a value to another as in:

15px * 0.33em\/px = 5em
15px * 0.33em / 1px = 5em

Complex Dimension (Horrible) Syntax

One might ask: "Why is the syntax of multi-dimensions so terrible?" and I would have to say that it very much looks like a quite legitimate question.

The CSS syntax allows an identifier like string to follow a number. That string supports backslashes, but not the asterisk nor the slash character (also no spaces if you wanted to also include a space before and after each asterisk and slash character.) There are many reasons for that, but with CSS Preprocessor we could anyway not distinguish between an asterisk used for multiplications, a slash used for divisions or as a separator, and spaces used to separate objects.

We may later want to support a string written between quotes such as in:

25"px * px"

But that should be viewed as an integer (25) followed by a string ("px * px"). I do not think that happens too often in CSS code, so it could be useful to support such and make it a lot easier to write complex dimensions. Plus, INTEGER + STRING can (and probably should) be written with a space in between (INTEGER + WHITESPACE + STRING) and that would not be taken as a dimension.

The origin of the * and / in complex dimensions comes from SASS.


Colors can be manipulated with some of the operators: add (+), subtract (-), multiply (*), divide (/), modulo (%), not(). However, all those manipulations make use of the RGB components. You may want to look into using HSL functions instead. These functions generally makes it easier to deal with colors.

Colors can be transformed to a Boolean value (See Booleans.) Black is false, whatever the alpha channel, any other color is true.

Note that we do not clamp color components until the time we have to output the result. So one can safely add two colors and then divide them by 2.0 to get the average and thus merge two colors together.

\[ { { \begin{pmatrix}r_1, g_1, b_1, a_1\end{pmatrix} + \begin{pmatrix}r_2, g_2, b_2, a_2\end{pmatrix} } \over 2 } = { \begin{pmatrix} { {r_1 + r_2} \over 2}, { {g_1 + g_2} \over 2 }, { {b_1 + b_2} \over 2 }, { {a_1 + a_2} \over 2 } \end{pmatrix} } \]

The same operation using the CSS Preprocessor syntax:

(red + blue) / 2

Note that color components are always saved as 32 bit floats so all operations are always viewed as floating point operations even if the product or division uses an integer and the color is defined using the rgb() function with integers. Also, this may throw you off since the operations may not give you the expected results unless you remember that rgb() components are all first divided by 255.0.

The supported operations are:

The color +/- number is expected to be used with a decimal number. If you add or subtract an integer, the color becomes white (+1) or becomes transparent (-1) unless the offset is zero. Note that the alpha channel is also affected.

\[ \begin{pmatrix} r + k, g + k, b + k, a + k \end{pmatrix} = \begin{pmatrix} r, g, b, a \end{pmatrix} + k \tag{Add Offset k to Color} \]

In most cases, it probably makes more sense to use frgba() to add the offset to only the color components. There is an example where the offset is 3 (which would be \(k = { 3 \over 255 }\) in the previous example) and does not affect the alpha channel:
chocolate + rgba(3, 3, 3, 0)
The || and && operators can also be used with other parameters than colors on either side of the operator.
All colors have an alpha channel and arithmetic operations may change the alpha channel making an otherwise opaque color semi-transparent. You may use various tricks to force a color to be opaque.
// if your color computation is not going to create a negative alpha
(color calculations...) + rgba(0, 0, 0, 1.0)
// when alpha may really be any value
rgb(color calculations...)
// or explicitly
rgba(color calculations..., 1.0)

Documentation of CSS Preprocessor.

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