using a variable in regex cpp

Question

I have a string:

string str1= "hello"; 
string str2="hel";

And I am doing this to see if str1 has str2 in it:

regex e = str2+"*"
 bool x=regex_match(str1,e); 
cout<<x;

I am getting this : error: conversion from 'std::__cxx11::basic_string' to non-scalar type 'std::__cxx11::regex {aka std::__cxx11::basic_regex}' requested regex e = str2+"";

How to use the variable in my regex. Can someone please tell me where am I going wrong? TIA.

See https://stackoverflow.com/questions/662918/how-do-i-concatenate-multiple-c-strings-on-one-line — Wiktor Stribiżew, Feb 22 '18 at 19:24
what types are `str1`, `str2`, `hello` and `hel`? How are they defined? Can you provide the complete code that produces the problem? — Galik, Feb 22 '18 at 19:25
@WiktorStribiżew that question is for concatenation of string! — hydra123, Feb 22 '18 at 19:26
Also note that the regex `hel*` _will_ match `hello`, but not for the reason you probably think. — CompuChip, Feb 22 '18 at 19:26
@hydra123 Right. But that is what you need. Concatenate the *string patterns* and then build the `regex` object. — Wiktor Stribiżew, Feb 22 '18 at 19:28
In your real code does `hello` have quotes round it or is it another variable? — Galik, Feb 22 '18 at 19:28
It is just hard to guess what the error is when the code you posted is not the same code that produces your error. — Galik, Feb 22 '18 at 19:30
While this is probably a learning exercise, it's worth pointing out that using a regular expression for this particular search is overkill. `str1.find(str2)` will tell you whether `str2` occurs anywhere in `str1`. — Pete Becker, Feb 22 '18 at 19:38

score 5 · Accepted Answer · answered Feb 22 '18 at 19:30

Look at reference about constructor of regex

template <class ST, class SA>
explicit basic_regex ( const basic_string<charT,ST,SA>& str, flag_type flags = 
 ECMAScript );

constructor is explicit so you should write

    regex e(str2+"*");

not

    regex e = str2+"*";

using a variable in regex cpp

1 Answers1