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# expr — Evaluate Expressions
## Overview
`expr` evaluates arithmetic, string, and logical expressions from the command
line and writes the result to standard output. It implements a recursive
descent parser with automatic type coercion between strings, numeric strings,
and integers.
**Source**: `expr/expr.c` (single file)
**Origin**: BSD 4.4, University of California, Berkeley
**License**: BSD-3-Clause
## Synopsis
```
expr expression
```
## Source Analysis
### Value Types
```c
enum value_type {
INTEGER, /* Pure integer (from arithmetic) */
NUMERIC_STRING, /* String that looks like a number */
STRING, /* General string */
};
struct value {
enum value_type type;
union {
intmax_t ival;
char *sval;
};
};
```
`expr` automatically coerces between types during operations. A value
like `"42"` starts as `NUMERIC_STRING` and is promoted to `INTEGER` for
arithmetic.
### Parser Architecture
`expr` uses a recursive descent parser with operator precedence:
```
parse_expr()
└── parse_or() /* | operator (lowest precedence) */
└── parse_and() /* & operator */
└── parse_compare() /* =, !=, <, >, <=, >= */
└── parse_add() /* +, - */
└── parse_mul() /* *, /, % */
└── parse_primary() /* atoms, ( expr ), : regex */
```
### Operators
#### Arithmetic Operators
| Operator | Description | Example |
|----------|-------------|---------|
| `+` | Addition | `expr 2 + 3` → `5` |
| `-` | Subtraction | `expr 5 - 2` → `3` |
| `*` | Multiplication | `expr 4 \* 3` → `12` |
| `/` | Integer division | `expr 10 / 3` → `3` |
| `%` | Modulo | `expr 10 % 3` → `1` |
#### Comparison Operators
| Operator | Description | Example |
|----------|-------------|---------|
| `=` | Equal | `expr abc = abc` → `1` |
| `!=` | Not equal | `expr abc != def` → `1` |
| `<` | Less than | `expr 1 \< 2` → `1` |
| `>` | Greater than | `expr 2 \> 1` → `1` |
| `<=` | Less or equal | `expr 1 \<= 1` → `1` |
| `>=` | Greater or equal | `expr 2 \>= 1` → `1` |
Comparisons between numeric strings use numeric ordering; otherwise
locale-aware string comparison (`strcoll`) is used.
#### Logical Operators
| Operator | Description | Example |
|----------|-------------|---------|
| `\|` | OR (short-circuit) | `expr 0 \| 5` → `5` |
| `&` | AND (short-circuit) | `expr 1 \& 2` → `1` |
#### String/Regex Operators
| Operator | Description | Example |
|----------|-------------|---------|
| `:` | Regex match | `expr hello : 'hel\(.*\)'` → `lo` |
| `match` | Same as `:` | `expr match hello 'h.*'` |
| `substr` | Substring | `expr substr hello 2 3` → `ell` |
| `index` | Character position | `expr index hello l` → `3` |
| `length` | String length | `expr length hello` → `5` |
### Regex Matching
The `:` operator uses POSIX basic regular expressions (`regcomp` with
`REG_NOSUB` or group capture):
```c
/* expr STRING : REGEX */
/* Returns captured \(...\) group or match length */
```
If the regex contains `\(...\)`, the captured substring is returned.
Otherwise, the length of the match is returned.
### Overflow Checking
All arithmetic operations check for integer overflow:
```c
static intmax_t
safe_add(intmax_t a, intmax_t b)
{
if ((b > 0 && a > INTMAX_MAX - b) ||
(b < 0 && a < INTMAX_MIN - b))
errx(2, "integer overflow");
return a + b;
}
```
### Locale Awareness
String comparisons use `strcoll(3)` for locale-correct ordering:
```c
/* Compare as strings using locale collation */
result = strcoll(left->sval, right->sval);
```
## System Calls Used
| Syscall | Purpose |
|---------|---------|
| `regcomp(3)` / `regexec(3)` | Regular expression matching |
| `strcoll(3)` | Locale-aware string comparison |
## Examples
```sh
# Arithmetic
expr 2 + 3 # → 5
expr 10 / 3 # → 3
expr 7 % 4 # → 3
# String length
expr length "hello" # → 5
# Regex match (capture group)
expr "hello-world" : 'hello-\(.*\)' # → world
# Regex match (length)
expr "hello" : '.*' # → 5
# Substring
expr substr "hello" 2 3 # → ell
# Index (first occurrence)
expr index "hello" "lo" # → 3
# Comparisons
expr 42 = 42 # → 1
expr abc \< def # → 1
# Logical OR (returns first non-zero/non-empty)
expr 0 \| 5 # → 5
expr "" \| alt # → alt
# In shell scripts
count=$(expr $count + 1)
```
## Exit Codes
| Code | Meaning |
|------|---------|
| 0 | Expression is neither null nor zero |
| 1 | Expression is null or zero |
| 2 | Expression is invalid |
| 3 | Internal error |
## Differences from GNU expr
- No `--help` or `--version`
- Identical POSIX semantics for `:` operator
- Locale-aware string comparison by default
- Overflow results in error, not wraparound
|
