STL String & Regex

Question 1

How to replace string?

Answer 1

1. find the position of start replacing
2. specify size of characters to be replaced
3. provide new string.

Example:

string data {“data 123 okay”};

auto pos = data.find( “123” );

data.replace( pos, sizeof(“123”) - 1, “56789” );

Question 2

How to create string from one char?

Answer 2

string s (1, ‘x’ );

or

string s{‘x’};

Question 3

string s1(“love”, 5 );

std::cout << s1.length();

what is the output?

Answer 3

5 it is “love\0”

Question 4

Is following code work?

string sql = “Select * from Order “

“Where ID = ‘A001’ “

Answer 4

Yes and what is in the sql string is

“Select * from Order Where ID = ‘A001’ “

Question 5

What is Raw string literals?

Answer 5

string stringInQuote = R”(This is a “string”)”;

R”( )”

Question 6

regex “ab+c” mean?

Answer 6

+ mean 1 or more so the one that valid will be start with ‘a’ and ‘b’ many then end with ‘c’ Ex: “abbbbbc” or “abbc”

Question 7

regex “ab*c+” mean?

Answer 7

* mean zero or more

+ mean one or more
so the valid one will be: “ac” “accc” “abcccc”

Question 8

What does “regex_match” do? and what is its syntax?

Answer 8

To test the whole input string match the pattern or not. It return yes or no.

regex r(“ab*c+”);

if ( regex_match( input_string, r ) then do something;

Question 9

In regex, what is () used for?

Answer 9

() parentheses is for precedence and capture group precedence “ax|yc” this is “ax” or “yc” so we can do “a(x|y)c” which is mean a then (x or y) and c capture group like “” then smatch the first group is in first and second group is in second or we can do “” the \d is \d for any digit the * for more. So if I do <34234><333> it is matched and first group is 34234 and second is 333

Question 10

In regex, what is “?”

Answer 10

0 or 1

Question 11

What is regex for date?

Answer 11

const regex r(“(\d{2}/\d{2}/\d{4})”); // this one is one capture group also with 3 capture groups const regex r(“(\d{2})/(\d{2})/(\d{4})”);

Question 12

How does regex_match work?

Answer 12

regex_match( input_string, sub_match, regex ); the the whole string match then return true, false if not.

sub_match is smatch class, is the result of capture group. the first sub_match is the whole match and next is each sub group.

for (i : sub_match )

cout << i << endl; or

you can say

sub_match[0]; // the whole match

sub_match[1]; // capture group 1

Question 13

How does regex_search work?

Answer 13

match part of the string.

the regex_match is for the whole string

regex_search( input_string, sub_match, regex );

sub_match.prefix() is the precedance that does not match sub_match.suffix() is the rest after match that does not match

Question 14

sregex_token_iterator is the typdef of?

Answer 14

regex_token_iterator

Question 15

What is regex_iterator?

Answer 15

iterator of found match regex.

Question 16

How to use regex_iterator?

Answer 16

const regex r(“\S+”); // anything not whitespace

string input(“aa bb cc dd”);

for (sregex_iterator i(input.begin(), input.end(), r), end; i != end; ++i) {

cout << (*i)[0] << endl;

}

the output is

aa bb cc dd

Question 17

Can we pass C-String to function that parameter is reference of string (string&)?

Answer 17

No because initial value of reference to non-const must be lvalue. the following is invalid syntax

void test (string& s);

test(“this is C-string”); // can’t compile

test( string(“This is rvalue”) // can’t compile

void test( const string& s);

test(“this is C-string”); // okay

test( string(“This is rvalue”) // okay