# expr

## About

Print the value of `EXPRESSION` to standard output

## Usage
```
expr [EXPRESSION]
expr [OPTIONS]
```

## After help

Print the value of `EXPRESSION` to standard output.  A blank line below
separates increasing precedence groups. 

`EXPRESSION` may be:

    ARG1 | ARG2       ARG1 if it is neither null nor 0, otherwise ARG2

    ARG1 & ARG2       ARG1 if neither argument is null or 0, otherwise 0

    ARG1 < ARG2       ARG1 is less than ARG2
    ARG1 <= ARG2      ARG1 is less than or equal to ARG2
    ARG1 = ARG2       ARG1 is equal to ARG2
    ARG1 != ARG2      ARG1 is unequal to ARG2
    ARG1 >= ARG2      ARG1 is greater than or equal to ARG2
    ARG1 > ARG2       ARG1 is greater than ARG2

    ARG1 + ARG2       arithmetic sum of ARG1 and ARG2
    ARG1 - ARG2       arithmetic difference of ARG1 and ARG2

    ARG1 * ARG2       arithmetic product of ARG1 and ARG2
    ARG1 / ARG2       arithmetic quotient of ARG1 divided by ARG2
    ARG1 % ARG2       arithmetic remainder of ARG1 divided by ARG2

    STRING : REGEXP   anchored pattern match of REGEXP in STRING

    match STRING REGEXP        same as STRING : REGEXP
    substr STRING POS LENGTH   substring of STRING, POS counted from 1
    index STRING CHARS         index in STRING where any CHARS is found, or 0
    length STRING              length of STRING
    + TOKEN                    interpret TOKEN as a string, even if it is a
                                 keyword like 'match' or an operator like '/'

    ( EXPRESSION )             value of EXPRESSION

Beware that many operators need to be escaped or quoted for shells.
Comparisons are arithmetic if both ARGs are numbers, else lexicographical.
Pattern matches return the string matched between \( and \) or null; if
\( and \) are not used, they return the number of characters matched or 0.

Exit status is `0` if `EXPRESSION` is neither null nor `0`, `1` if `EXPRESSION` is null
or `0`, `2` if `EXPRESSION` is syntactically invalid, and `3` if an error occurred.

Environment variables:
 - `EXPR_DEBUG_TOKENS=1`: dump expression's tokens
 - `EXPR_DEBUG_RPN=1`: dump expression represented in reverse polish notation
 - `EXPR_DEBUG_SYA_STEP=1`: dump each parser step
 - `EXPR_DEBUG_AST=1`: dump expression represented abstract syntax tree