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#### Options for Perl Regular Expressions

One of the following must always be set for perl regular expressions:

Element | Standardized | Effect when set
--- | --- | ---
ECMAScript | Yes | Specifies that the grammar recognized by the regular expression engine uses its normal semantics: that is the same as that given in the ECMA-262, ECMAScript Language Specification, Chapter 15 part 10, RegExp (Regular Expression) Objects (FWD.1). This is functionally identical to the Perl regular expression syntax. Boost.Regex also recognizes all of the perl-compatible `(?...)` extensions in this mode.
perl | No | As above.
normal | No | As above.
JavaScript | No | As above.
JScript | No | As above.

The following options may also be set when using perl-style regular expressions:

Element | Standardized | Effect when set
--- | --- | ---
icase | Yes | Specifies that matching of regular expressions against a character container sequence shall be performed without regard to case.
nosubs | Yes | Specifies that when a regular expression is matched against a character container sequence, then no sub-expression matches are to be stored in the supplied `match_results` structure.
optimize | Yes | Specifies that the regular expression engine should pay more attention to the speed with which regular expressions are matched, and less to the speed with which regular expression objects are constructed. Otherwise it has no detectable effect on the program output. This currently has no effect for Boost.Regex.
collate | Yes | Specifies that character ranges of the form `[a-b]` should be locale sensitive.
newline_alt | No | Specifies that the \n character has the same effect as the alternation operator |. Allows newline separated lists to be used as a list of alternatives.
no_except | No | Prevents `basic_regex` from throwing an exception when an invalid expression is encountered.
no_mod_m | No | Normally Boost.Regex behaves as if the Perl m-modifier is on: so the assertions ^ and $ match after and before embedded newlines respectively, setting this flags is equivalent to prefixing the expression with (?-m).
no_mod_s | No | Normally whether Boost.Regex will match "." against a newline character is determined by the match flag `match_dot_not_newline`. Specifying this flag is equivalent to prefixing the expression with `(?-s)` and therefore causes "." not to match a newline character regardless of whether `match_not_dot_newline` is set in the match flags.
mod_s | No | Normally whether Boost.Regex will match "." against a newline character is determined by the match flag `match_dot_not_newline`. Specifying this flag is equivalent to prefixing the expression with `(?s)` and therefore causes "." to match a newline character regardless of whether `match_not_dot_newline` is set in the match flags.
mod_x | No | Turns on the perl x-modifier: causes unescaped whitespace in the expression to be ignored.
no_empty_expressions | No | When set then empty expressions/alternatives are prohibited.
save_subexpression_location | No | When set then the locations of individual sub-expressions within the original regular expression string can be accessed via the `subexpression()` member function of `basic_regex`.