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# std::regex_constants::syntax_option_type

From cppreference.com
< cpp‎ | regex

Defined in header `<regex>` |  |
typedef /*unspecified*/ syntax_option_type; constexpr syntax_option_type icase = /*unspecified*/; constexpr syntax_option_type nosubs = /*unspecified*/; constexpr syntax_option_type optimize = /*unspecified*/; constexpr syntax_option_type collate = /*unspecified*/; constexpr syntax_option_type ECMAScript = /*unspecified*/; constexpr syntax_option_type basic = /*unspecified*/; constexpr syntax_option_type extended = /*unspecified*/; constexpr syntax_option_type awk = /*unspecified*/; constexpr syntax_option_type grep = /*unspecified*/; constexpr syntax_option_type egrep = /*unspecified*/; |  |

The `syntax_option_type` is a `BitmaskType` that contains options that govern how regular expressions behave.

The possible values for this type (`icase`, `optimize`, etc.) are duplicated inside std::basic_regex.

### [edit] Constants

Value | Effect(s)
`icase` | Character matching should be performed without regard to case.
`nosubs` | When performing matches, all marked sub-expressions `(expr)` are treated as non-marking sub-expressions `(?:expr)`. No matches are stored in the supplied std::regex_match structure and `mark_count()` is zero
`optimize` | Instructs the regular expression engine to make matching faster, with the potential cost of making construction slower. For example, this might mean converting a non-deterministic FSA to a deterministic FSA.
`collate` | Character ranges of the form "[a-b]" will be locale sensitive.
`ECMAScript` | Use the Modified ECMAScript regular expression grammar
`basic` | Use the basic POSIX regular expression grammar (grammar documentation).
`extended` | Use the extended POSIX regular expression grammar (grammar documentation).
`awk` | Use the regular expression grammar used by the awk utility in POSIX (grammar documentation)
`grep` | Use the regular expression grammar used by the grep utility in POSIX. This is effectively the same as the `basic` option with the addition of newline '\n' as an alternation separator.
`egrep` | Use the regular expression grammar used by the grep utility, with the -E option, in POSIX. This is effectively the same as the `extended` option with the addition of newline '\n' as an alternation separator in addtion to '|'.

At most one grammar option must be chosen out of `ECMAScript`, `basic`, `extended`, `awk`, `grep`, `egrep`. If no grammar is chosen, `ECMAScript` is assumed to be selected. The other options serve as modifiers, such that std::regex("meow", std::regex::icase) is equivalent to std::regex("meow", std::regex::ECMAScript|std::regex::icase)

### [edit] Notes

Because POSIX uses "leftmost longest" matching rule (the longest matching subsequence is matched, and if there are several such subsequences, the first one is matched), it is not suitable, for example, for parsing markup languages: a POSIX regex such as "<tag[^>]*>.*</tag>" would match everything from the first "<tag" to the last "</tag>", including every "</tag>" and "<tag>" inbetween. On the other hand, ECMAScript supports non-greedy matches, and the ECMAScript regex "<tag[^>]*>.*?</tag>" would match only until the first closing tag.

In C++11, these constants were specified with redundant keyword `static`, which was removed by C++14 via LWG issue 2053

### [edit] Example

Illustrates the difference in the matching algorithm between ECMAScript and POSIX regular expressions

Run this code

    #include <iostream>
    #include <string>
    #include <regex>

    int main()
    {
        std::string str = "zzxayyzz";
        std::regex re1(".*(a|xayy)"); // ECMA
        std::regex re2(".*(a|xayy)", std::regex::extended); // POSIX

        std::cout << "Searching for .*(a|xayy) in zzxayyzz:\n";
        std::smatch m;
        std::regex_search(str, m, re1);
        std::cout << " ECMA (depth first search) match: " << m[0] << '\n';
        std::regex_search(str, m, re2);
        std::cout << " POSIX (leftmost longest)  match: " << m[0] << '\n';
    }

Output:

    Searching for .*(a|xayy) in zzxayyzz:
     ECMA (depth first search) match: zzxa
     POSIX (leftmost longest)  match: zzxayy

### [edit] See also

basic_regex (C++11) | regular expression object (class template) [edit]

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