expr: Fix regex anchor matching behavior with REGEX_OPTION_SINGLELINE

The previously used `REGEX_OPTION_NONE` allowed anchors (^) and ($) to match across newlines.

New anchor behaviors:
- `^` matches the start of the entire string (`\A`)
- `$` matches the end of the entire string (`\Z`)

src/uu/expr/src/syntax_tree.rs

@@ -154,7 +154,7 @@ impl StringOp {
                 let re_string = format!("{prefix}{right}");
                 let re = Regex::with_options(
                     &re_string,
-                    RegexOptions::REGEX_OPTION_NONE,
+                    RegexOptions::REGEX_OPTION_SINGLELINE,
                     Syntax::grep(),
                 )
                 .map_err(|_| ExprError::InvalidRegexExpression)?;