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# Boost.Regex

## FAQ

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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.

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 regex++ expression compiler, so to pass the regular expression \d+ to regex++, you need to embed "\\d+" in your code. Likewise to match a literal backslash you will need to embed "\\\\" in your code.

Q. Why does using parenthesis in a POSIX regular expression change the result of a match?

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"

where as

    "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:

    "0*([1-9][0-9]*)"

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 setting the regex_constants::collate compile time flag. However if you set a non-default compile time flag - for example regex_constants::extended or regex_constants::basic, then locale dependent collation will be enabled, this also applies to the POSIX API functions which use either regex_constants::extended or 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::bad_expression can be thrown by basic_regex when compiling a regular expression, std::runtime_error can be thrown when a call to basic_regex::imbue tries to open a message catalogue that doesn't exist, or when a call to regex_search or regex_match results in an "everlasting" search, or when a call to RegEx::GrepFiles or 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.

Revised 24 Oct 2003

© Copyright John Maddock 1998- 2003

Use, modification and distribution are subject to the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)