Several operations, such as [`nix-env --query`](./nix-env/query.md) and [`nix-env --install`](./nix-env/install.md), take a list of *arguments* that specify the packages on which to operate.
Packages are identified based on a `name` part and a `version` part of a [symbolic derivation name](@docroot@/language/derivations.md#attr-names):
-`name`: Everything up to but not including the first dash (`-`) that is *not* followed by a letter.
-`version`: The rest, excluding the separating dash.
> **Example**
>
> `nix-env` parses the symbolic derivation name `apache-httpd-2.0.48` as:
>
> ```json
> {
> "name": "apache-httpd",
> "version": "2.0.48"
> }
> ```
> **Example**
>
> `nix-env` parses the symbolic derivation name `firefox.*` as:
>
> ```json
> {
> "name": "firefox.*",
> "version": ""
> }
> ```
The `name` parts of the *arguments* to `nix-env` are treated as extended regular expressions and matched against the `name` parts of derivation names in the package source.
The match is case-sensitive.
The regular expression can optionally be followed by a dash (`-`) and a version number; if omitted, any version of the package will match.
For details on regular expressions, see [**regex**(7)](https://linux.die.net/man/7/regex).
> **Example**
>
> Common patterns for finding package names with `nix-env`:
>
> - `firefox`
>
> Matches the package name `firefox` and any version.
>
> - `firefox-32.0`
>
> Matches the package name `firefox` and version `32.0`.
>
> - `gtk\\+`
>
> Matches the package name `gtk+`.
> The `+` character must be escaped using a backslash (`\`) to prevent it from being interpreted as a quantifier, and the backslash must be escaped in turn with another backslash to ensure that the shell passes it on.
>
> - `.\*`
>
> Matches any package name.
> This is the default for most commands.
>
> - `'.*zip.*'`
>
> Matches any package name containing the string `zip`.
> Note the dots: `'*zip*'` does not work, because in a regular expression, the character `*` is interpreted as a quantifier.
>
> - `'.*(firefox|chromium).*'`
>
> Matches any package name containing the strings `firefox` or `chromium`.
