adds latex lookup

Author: rokke-git

NEWS.md

@@ -11,6 +11,8 @@ New language features
   - `ᵅ` (U+U+1D45), `ᵋ` (U+1D4B), `ᶲ` (U+1DB2), `˱` (U+02F1), `˲` (U+02F2), and `ₔ` (U+2094) can now also be used as
     operator suffixes, accessible as `\^alpha`, `\^epsilon`, `\^ltphi`, `\_<`, `\_>`, and `\_schwa` at the REPL
     ([#60285]).
+  - Latex expansions can now be searched like `\?search<tab>` to show all symbols *containing* rather than starting
+    with the search string ([#61464]).
   - The `@label` macro can now create labeled blocks that can be exited early with `break name [value]`. Use
     `@label name expr` for named blocks or `@label expr` for anonymous blocks. Anonymous `@label` blocks
     participate in the default break scope: a plain `break` or `break _` exits the innermost breakable scope,

stdlib/REPL/src/REPLCompletions.jl

@@ -879,8 +879,9 @@ function bslash_completions(string::String, pos::Int, hint::Bool=false)
         # return possible matches; these cannot be mixed with regular
         # Julian completions as only latex / emoji symbols contain the leading \
         symbol_dict = startswith(s, "\\:") ? emoji_symbols : latex_symbols
-        namelist = Iterators.filter(k -> startswith(k, s), keys(symbol_dict))
-        completions = Completion[BslashCompletion(name, "$(symbol_dict[name]) $name") for name in sort!(collect(namelist))]
+        filt = startswith(s, "\\?") ? k -> contains(k, s[3:end]) : k -> startswith(k, s)
+        namelist = Iterators.filter(filt, keys(symbol_dict)) |> collect |> sort!
+        completions = Completion[BslashCompletion(name, "$(symbol_dict[name]) $name") for name in namelist]
         return (true, (completions, slashpos:pos, true))
     end
     return (false, (Completion[], 1:0, false))