libexpr: Replace regex engine with boost::regex

This avoids C++'s standard library regexes, which aren't the same
across platforms, and have many other issues, like using stack
so much that they stack overflow when processing a lot of data.

To avoid backwards and forward compatibility issues, regexes are
processed using a function converting libstdc++ regexes into Boost
regexes, escaping characters that Boost needs to have escaped, and
rejecting features that Boost has and libstdc++ doesn't.

Related context:

- Original failed attempt to use `boost::regex` in CppNix, failed due to
  boost icu dependency being large (disabling ICU is no longer necessary
  because linking ICU requires using a different header file,
  `boost/regex/icu.hpp`): https://github.com/NixOS/nix/pull/3826

- An attempt to use PCRE, rejected due to providing less backwards
  compatibility with `std::regex` than `boost::regex`:
  https://github.com/NixOS/nix/pull/7336

- Second attempt to use `boost::regex`, failed due to `}` regex failing
  to compile (dealt with by writing a wrapper that parses a regular
  expression and escapes `}` characters):
  https://github.com/NixOS/nix/pull/7762

Closes #34. Closes #476.

Change-Id: Ieb0eb9e270a93e4c7eed412ba4f9f96cb00a5fa4

2 files changed, 41 insertions, 0 deletions

diff --git a/doc/manual/change-authors.yml b/doc/manual/change-authors.yml
index e18abada1..d9303a747 100644
--- a/doc/manual/change-authors.yml
+++ b/doc/manual/change-authors.yml
@@ -129,6 +129,10 @@ roberth:
   display_name: Robert Hensing
   github: roberth
 
+sugar:
+  forgejo: sugar
+  github: sugar700
+
 thufschmitt:
   display_name: Théophane Hufschmitt
   github: thufschmitt
diff --git a/doc/manual/rl-next/boost-regex.md b/doc/manual/rl-next/boost-regex.md
new file mode 100644
index 000000000..c541434d0
--- /dev/null
+++ b/doc/manual/rl-next/boost-regex.md
@@ -0,0 +1,37 @@
+---
+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
+                )
+              )
+    [ ]