Problems with regex in C++

Question

I want to match following string but receive always false and match is empty. Only if the expression is equal to the string the return value is true. My regex seems to be correct. What am I doing wrong?

#include <regex>

using namespace std;

int main()
{
   regex rgx("use_gui=((true|false)); state=((bla|blub|blob))");
   cmatch cm;
   string s("use_gui=false; state=bla");
   bool ret = regex_match(s.c_str(), cm, rgx);

}

See this if you're using gcc: http://stackoverflow.com/a/12665408/873025 — Felix Glas, Nov 29 '13 at 15:12
It matches for me with G++ 4.8.1 locally and at ideone: ideone.com/2zIz4u — Magnus Hoff, Nov 29 '13 at 15:27

Julien B. · Accepted Answer · 2013-11-29T15:30:27.770

2

Consider removing the extra parenthesis:

regex rgx("use_gui=(true|false); state=(bla|blub|blob)");

This fixed it for me. (Thanks @Lightness Races in Orbit for showing me coliru)

edited Nov 29 '13 at 15:30

answered Nov 29 '13 at 15:15

Julien B.

88
6

I didn't say the code is not correct. I don't match the string. – alex555 Nov 29 '13 at 15:17
Well, he does have more () than necessary (but that shouldn't prevent the match). – James Kanze Nov 29 '13 at 15:17
It matches [almost correctly](http://coliru.stacked-crooked.com/a/a63b9a6c02362da5) but there _is_ something up – Lightness Races in Orbit Nov 29 '13 at 15:18
I just ran the code in Visual Studio 2012. It also matches for me. – Nov 29 '13 at 15:19
@LightnessRacesinOrbit, am I right in thinking it executes perfectly on your coliru link? – OGHaza Nov 29 '13 at 15:26
@OGHaza: No, as I just said, there is still something up. The `=` shouldn't be part of the matches. But it _does_ match so the OP is seeing something else. – Lightness Races in Orbit Nov 29 '13 at 15:27
@LightnessRacesinOrbit, ah right, I couldn't see what was up before – OGHaza Nov 29 '13 at 15:28
2

@alex555: Not in the code you showed us, you didn't. Are you saying you posted a question that did not reflect the code you were _actually_ running? What a waste of our time :( – Lightness Races in Orbit Nov 29 '13 at 15:30
@LightnessRacesinOrbit: It's not. Before posting I had a `\s` in the middle of the regex string. I admit, that the code is not exactly the same, but usually `\s` is a blank, `use_gui=(true|false);\sstate=(bla|blub|blob)` returns false – alex555 Nov 29 '13 at 15:47
This is why it's important to post _exactly_ the code that produced a failure in your environment. :-( – Tim Pierce Nov 29 '13 at 15:50
@alex555: Well that is completely different! – Lightness Races in Orbit Nov 29 '13 at 16:47

score 1 · Answer 2 · answered Nov 29 '13 at 15:26

1

The code you've written matches correctly for me and matches the string. I recommend not using the double parentheses, which are not necessary and may be confusing the regex engine that you're using:

int main()
{
   regex rgx("use_gui=(true|false); state=(bla|blub|blob)");
   cmatch cm;
   string s("use_gui=false; state=bla");
   bool ret = regex_match(s.c_str(), cm, rgx);
   cout << "matches: " << cm.size() << endl;
   for (cmatch::iterator p = cm.begin(); p != cm.end(); ++p) {
     cout << "match: " << *p << endl;
   }
}

On my machine (OSX 10.9, XCode) this produces:

matches: 3
match: use_gui=false; state=bla
match: false
match: bla

answered Nov 29 '13 at 15:26

Tim Pierce

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`for (auto p : cm) cout << "match: " << p << '\n';` is all you need! – Lightness Races in Orbit Nov 29 '13 at 15:28
Ooh, that's lovely, thanks. (My C++ experience comes mostly from pre-C++11, so I'm still catching up to all of this newfangled `auto` stuff you kids are using.) – Tim Pierce Nov 29 '13 at 15:29
I am not an `auto` fanboy by _any_ stretch of the imagination, but to me this is one of the rare GOOD uses. :-) – Lightness Races in Orbit Nov 29 '13 at 15:30

score 1 · Answer 3 · edited May 23 '17 at 10:25

1

So your regex is basically valid, despite the superfluous capture groups. There must be something you're not showing us.

Unfortunately, in GCC 4.8 you get a = at the start of each match, but regexes are fixed properly in GCC 4.9.

edited May 23 '17 at 10:25

Community

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answered Nov 29 '13 at 15:46

Lightness Races in Orbit

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Problems with regex in C++

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