Switch to using pcre::regex instead of boost::regex

Drop dependency on Boost.Regex library and use PCRE-based regex class
instead.

Using PCRE-based implementation is advantageous compared to using
std::regex because it has a much better performance and also has more
features, notably it supports "?<=" look-behind assertions not supported
by the standard class, that are used in the existing code.

The only regex change needed was the one replacing "\l", which is not
recognized by PCRE (nor std::regex) as a character class shorthand with
the explicit use "[:lower:]".

.github/workflows/ci.yml

@@ -204,23 +204,6 @@ jobs:
         if: steps.cache-local.outputs.cache-hit != 'true'
         run: ./install_wxpdfdoc.sh
 
-      - name: Build Boost.Regex
-        if: matrix.autotools
-        run: |
-          # Define flags to avoid warnings in Boost code we don't care about.
-          boost_cxxflags="-Wno-deprecated-declarations -Wno-register -DBOOST_NO_AUTO_PTR -DBOOST_NO_STD_ALLOCATOR"
-          if [ "${{ matrix.compiler }}" = clang ]; then
-            boost_cxxflags="${boost_cxxflags} -Wno-parentheses-equality"
-          fi
-
-          for lib in regex; do
-            cd /opt/lmi/third_party/src/boost/libs/${lib}/src
-            echo "Compiling $lib sources"
-            ${CXX-${LMI_COMPILER}} -std=c++17 -fPIC $(echo ${boost_cxxflags}) -I../../.. -c *.cpp
-            echo "Creating libboost_${lib}.a from" *.o
-            ar rc /opt/lmi/local/${LMI_COMPILER}_${LMI_TRIPLET}/lib/libboost_${lib}.a *.o
-          done
-
       - name: Configure lmi
         if: matrix.autotools
         run: |

Makefile.am

@@ -1132,7 +1132,6 @@ noinst_HEADERS = \
     authenticity.hpp \
     basic_tables.hpp \
     basic_values.hpp \
-    boost_regex.hpp \
     bourn_cast.hpp \
     cache_file_reads.hpp \
     calendar_date.hpp \