Revert "libexpr: Replace regex engine with boost::regex"

This reverts commit 447212fa65a80180150b265411924cc638a2c52c.

Reason for revert: Regression in eval behaviour bug-compatibility.

Expected behaviour (Nix 2.18.5, macOS and Linux [libstdc++/libc++]):

```
nix-repl> builtins.match "\\.*(.*)" ".keep"
[ "keep" ]

nix-repl> builtins.match "(\\.*)(.*)" ".keep"
[ "." "keep" ]
```

Actual behaviour (boost::regex):

```
nix-repl> builtins.match "\\.*(.*)" ".keep"
[ ".keep" ]

nix-repl> builtins.match "(\\.*)(.*)" ".keep"
[
  "."
  "keep"
]
```

Bug: https://git.lix.systems/lix-project/lix/issues/483
Change-Id: Id462eb8586dcd54856cf095f09b3e3a216955b60

1 files changed, 0 insertions, 37 deletions

diff --git a/doc/manual/rl-next/boost-regex.md b/doc/manual/rl-next/boost-regex.md
deleted file mode 100644
index c541434d0..000000000
--- a/doc/manual/rl-next/boost-regex.md
+++ /dev/null
@@ -1,37 +0,0 @@
----
-synopsis: Replace regex engine with boost::regex
-issues: [fj#34, fj#476]
-cls: [1821]
-category: Fixes
-credits: [sugar]
----
-
-Previously, the C++ standard regex expression library was used, the
-behaviour of which varied depending on the platform. This has been
-replaced with the Boost regex library, which works identically across
-platforms.
-
-The visible behaviour of the regex functions doesn't change. While
-the new library has more features, Lix will reject regular expressions
-using them.
-
-This also fixes regex matching reporting stack overflow when matching
-on too much data.
-
-Before:
-
-    nix-repl> builtins.match ".*" (
-                builtins.concatStringsSep "" (
-                  builtins.genList (_: "a") 1000000
-                )
-              )
-    error: stack overflow (possible infinite recursion)
-
-After:
-
-    nix-repl> builtins.match ".*" (
-                builtins.concatStringsSep "" (
-                  builtins.genList (_: "a") 1000000
-                )
-              )
-    [ ]