Q. I can't get regex++ to work with escape characters, what's going on?
A. If you embed regular expressions in C++ code, then remember that escape characters are processed twice: once by the C++ compiler, and once by the Boost.Regex expression compiler, so to pass the regular expression \d+ to Boost.Regex, you need to embed "\d+" in your code. Likewise to match a literal backslash you will need to embed "\\" in your code.
Q. No matter what I do regex_match always returns false, what's going on?
A. The algorithm regex_match only succeeds if the expression matches all of the text, if you want to find a sub-string within the text that matches the expression then use regex_search instead.
Q. Why does using parenthesis in a POSIX regular expression change the result of a match?
A. For POSIX (extended and basic) regular expressions, but not for perl regexes, parentheses don't only mark; they determine what the best match is as well. When the expression is compiled as a POSIX basic or extended regex then Boost.Regex follows the POSIX standard leftmost longest rule for determining what matched. So if there is more than one possible match after considering the whole expression, it looks next at the first sub-expression and then the second sub-expression and so on. So...
"(0*)([0-9]*)" against "00123" would produce $1 = "00" $2 = "123"
"0*([0-9])*" against "00123" would produce $1 = "00123"
If you think about it, had $1 only matched the "123", this would be "less good" than the match "00123" which is both further to the left and longer. If you want $1 to match only the "123" part, then you need to use something like:
as the expression.
Q. Why don't character ranges work properly (POSIX mode only)?
A. The POSIX standard specifies that character
range expressions are locale sensitive - so for example the expression [A-Z]
will match any collating element that collates between 'A' and 'Z'. That
means that for most locales other than "C" or "POSIX",
[A-Z] would match the single character 't' for example, which is not what
most people expect - or at least not what most people have come to expect
from regular expression engines. For this reason, the default behaviour of
Boost.Regex (perl mode) is to turn locale sensitive collation off by not
compile time flag. However if you set a non-default compile time flag - for
then locale dependent collation will be enabled, this also applies to the
POSIX API functions which use either
regex_constants::basic internally. [Note - when
regex_constants::nocollate in effect, the library behaves
"as if" the LC_COLLATE locale category were always "C",
regardless of what its actually set to - end note].
Q. Why are there no throw specifications on any of the functions? What exceptions can the library throw?
A. Not all compilers support (or honor)
throw specifications, others support them but with reduced efficiency. Throw
specifications may be added at a later date as compilers begin to handle
this better. The library should throw only three types of exception: [boost::regex_error]
can be thrown by
basic_regex when compiling a regular
std::runtime_error can be thrown when a call
basic_regex::imbue tries to open a message catalogue
that doesn't exist, or when a call to
regex_match results in an "everlasting"
search, or when a call to
RegEx::FindFiles tries to open a file that cannot
be opened, finally
std::bad_alloc can be thrown by just about any
of the functions in this library.
Q. Why can't I use the "convenience" versions of regex_match / regex_search / regex_grep / regex_format / regex_merge?
A. These versions may or may not be available depending upon the capabilities of your compiler, the rules determining the format of these functions are quite complex - and only the versions visible to a standard compliant compiler are given in the help. To find out what your compiler supports, run <boost/regex.hpp> through your C++ pre-processor, and search the output file for the function that you are interested in. Note however, that very few current compilers still have problems with these overloaded functions.