c++11/regex - search for exact string, escape [duplicate]

Question

Say you have a string which is provided by the user. It can contain any kind of character. Examples are:

std::string s1{"hello world"); std::string s1{".*"); std::string s1{"*{}97(}{.}}\\testing___just a --%#$%# literal%$#%^"}; ...

Now I want to search in some text for occurrences of >> followed by the input string s1 followed by <<. For this, I have the following code:

std::string input; // the input text std::regex regex{">> " + s1 + " <<"}; if (std::regex_match(input, regex)) { // add logic here }

This works fine if s1 did not contain any special characters. However, if s1 had some special characters, which are recognized by the regex engine, it doesn't work.

How can I escape s1 such that std::regex considers it as a literal, and therefore does not interpret s1? In other words, the regex should be:

std::regex regex{">> " + ESCAPE(s1) + " <<"};

Is there a function like ESCAPE() in std?

important I simplified my question. In my real case, the regex is much more complex. As I am only having troubles with the fact the s1 is interpreted, I left these details out.

Is there a particular reason for using regular expressions? This can be done with string.find — Austin Brunkhorst
– Austin Brunkhorst, Commented Oct 22, 2016 at 18:11
Why are you using the input string (s1) as a regex?? Perhaps something like std::regex Regex{">>*<<"}; would be better? — ForceBru
– ForceBru, Commented Oct 22, 2016 at 18:12
@Austin Brunkhorst Yes, I need regular expressions in my case. I simplified the question, as I am only struggling with the fact that the string is interpreted by the engine. In my real case, the regex is more complex. — Karel Demeester
– Karel Demeester, Commented Oct 22, 2016 at 18:12
@ForceBru There are two inputs: s1 and some text. The string s1 is used to dynamically construct the regex, which is used to search in the input text. — Karel Demeester
– Karel Demeester, Commented Oct 22, 2016 at 18:14
This is related to "How to escape a string for use in Boost Regex" since std::regex was largely based on boost::regex. You might consult that question for an answer. — Cornstalks
– Cornstalks, Commented Oct 22, 2016 at 18:40

Austin Brunkhorst · Accepted Answer · 2016-10-22 18:40:43Z

12

You will have to escape all special characters in the string with \. The most straightforward approach would be to use another expression to sanitize the input string before creating the expression regex.

// matches any characters that need to be escaped in RegEx std::regex specialChars { R"([-[\]{}()*+?.,\^$|#\s])" }; std::string input = ">> "+ s1 +" <<"; std::string sanitized = std::regex_replace( input, specialChars, R"(\$&)" ); // "sanitized" can now safely be used in another expression

answered Oct 22, 2016 at 18:40

Austin Brunkhorst

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11 Comments

Cornstalks Over a year ago

Including ^ makes sense, but you've escaped it with a backslash. I'm curious why you've escaped it with a backslash in this situation. Also, you've included \s, which matches whitespace, but I'm not sure why you'd need that (maybe newline handling? I dunno; I can't remember how std::regex handles newlines and whether escaping them or not makes a difference). And in my previous comment I tried to say \ should be included in specialChars too, but Markdown ate it.

Karel Demeester Over a year ago

Also about #, should that be escaped?

Karel Demeester Over a year ago

And ,? Why should that be escaped?

user541686 Over a year ago

Why don't you escape backslash?

c00000fd Over a year ago

All one needs to escape with ` \ ` are these characters: [\^$.|?*+(){}. As per.

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