Make parse simple patterns ignore fields with placeholder (_) (#15873)

# Description
Simple `parse` patterns let you quickly put together simple parsers, but
sometimes you aren't actually interested in some of the output (such as
variable whitespace). This PR lets you use `{_}` to discard part of the
input.

Example:
```nushell
"hello world" | parse "{foo} {_}"
# => ╭───┬───────╮
# => │ # │  foo  │
# => ├───┼───────┤
# => │ 0 │ hello │
# => ╰───┴───────╯
```

here's a simple parser for the `apropops` using the `_` placeholder to
discard the variable whitespace, without needing to resort to a full
regex pattern:

```nushell
apropos linux | parse "{name} ({section}) {_}- {topic}"
# => ╭───┬───────────────────────────────────────┬─────────┬─────────────────────────────────────────────────────────────────────╮
# => │ # │                 name                  │ section │                                topic                                │
# => ├───┼───────────────────────────────────────┼─────────┼─────────────────────────────────────────────────────────────────────┤
# => │ 0 │ PAM                                   │ 8       │ Pluggable Authentication Modules for Linux                          │
# => │ 1 │ aarch64-linux-gnu-addr2line           │ 1       │ convert addresses or symbol+offset into file names and line numbers │
# => │ 2 │ ...                                   │ ...     │ ...                                                                 │
# => │ 3 │ xcb_selinux_set_window_create_context │ 3       │ (unknown subject)                                                   │
# => │ 4 │ xorriso-dd-target                     │ 1       │ Device evaluator and disk image copier for GNU/Linux                │
# => ╰───┴───────────────────────────────────────┴─────────┴─────────────────────────────────────────────────────────────────────╯
```

# User-Facing Changes
* `parse` simple patterns can now discard input using `{_}`

# Tests + Formatting
N/A

# After Submitting
N/A

crates/nu-command/src/strings/parse.rs

@@ -45,6 +45,13 @@ impl Command for Parse {
                     "bar" => Value::test_string("there"),
                 })])),
             },
+            Example {
+                description: "Parse a string, ignoring a column with _",
+                example: "\"hello world\" | parse \"{foo} {_}\"",
+                result: Some(Value::test_list(vec![Value::test_record(record! {
+                    "foo" => Value::test_string("hello"),
+                })])),
+            },
             Example {
                 description: "This is how the first example is interpreted in the source code",
                 example: "\"hi there\" | parse --regex '(?s)\\A(?P<foo>.*?) (?P<bar>.*?)\\z'",
@@ -277,9 +284,17 @@ fn build_regex(input: &str, span: Span) -> Result<String, ShellError> {
         }
 
         if !column.is_empty() {
-            output.push_str("(?P<");
+            output.push_str("(?");
+            if column == "_" {
+                // discard placeholder column(s)
+                output.push(':');
+            } else {
+                // create capture group for column
+                output.push_str("P<");
                 output.push_str(&column);
-            output.push_str(">.*?)");
+                output.push('>');
+            }
+            output.push_str(".*?)");
         }
 
         if before.is_empty() && column.is_empty() {

crates/nu-command/tests/commands/parse.rs

@@ -7,7 +7,7 @@ mod simple {
 
     #[test]
     fn extracts_fields_from_the_given_the_pattern() {
-        Playground::setup("parse_test_1", |dirs, sandbox| {
+        Playground::setup("parse_test_simple_1", |dirs, sandbox| {
             sandbox.with_files(&[Stub::FileWithContentToBeTrimmed(
                 "key_value_separated_arepa_ingredients.txt",
                 r#"
@@ -35,25 +35,19 @@ mod simple {
 
     #[test]
     fn double_open_curly_evaluates_to_a_single_curly() {
-        Playground::setup("parse_test_regex_2", |dirs, _sandbox| {
-            let actual = nu!(
-                cwd: dirs.test(), pipeline(
+        let actual = nu!(pipeline(
             r#"
                 echo "{abc}123"
                 | parse "{{abc}{name}"
                 | get name.0
             "#
         ));
-
         assert_eq!(actual.out, "123");
-        })
     }
 
     #[test]
     fn properly_escapes_text() {
-        Playground::setup("parse_test_regex_3", |dirs, _sandbox| {
-            let actual = nu!(
-                cwd: dirs.test(), pipeline(
+        let actual = nu!(pipeline(
             r#"
                 echo "(abc)123"
                 | parse "(abc){name}"
@@ -62,14 +56,11 @@ mod simple {
         ));
 
         assert_eq!(actual.out, "123");
-        })
     }
 
     #[test]
     fn properly_captures_empty_column() {
-        Playground::setup("parse_test_regex_4", |dirs, _sandbox| {
-            let actual = nu!(
-                cwd: dirs.test(), pipeline(
+        let actual = nu!(pipeline(
             r#"
                 echo ["1:INFO:component:all is well" "2:ERROR::something bad happened"]
                 | parse "{timestamp}:{level}:{tag}:{entry}"
@@ -79,14 +70,11 @@ mod simple {
         ));
 
         assert_eq!(actual.out, "something bad happened");
-        })
     }
 
     #[test]
     fn errors_when_missing_closing_brace() {
-        Playground::setup("parse_test_regex_5", |dirs, _sandbox| {
-            let actual = nu!(
-                cwd: dirs.test(), pipeline(
+        let actual = nu!(pipeline(
             r#"
                 echo "(abc)123"
                 | parse "(abc){name"
@@ -99,7 +87,22 @@ mod simple {
                 .err
                 .contains("Found opening `{` without an associated closing `}`")
         );
-        })
+    }
+
+    #[test]
+    fn ignore_multiple_placeholder() {
+        let actual = nu!(pipeline(
+            r#"
+                echo ["1:INFO:component:all is well" "2:ERROR::something bad happened"]
+                | parse "{_}:{level}:{_}:{entry}"
+                | to json -r
+            "#
+        ));
+
+        assert_eq!(
+            actual.out,
+            r#"[{"level":"INFO","entry":"all is well"},{"level":"ERROR","entry":"something bad happened"}]"#
+        );
     }
 }