Prevent interpretation of special characters in std::regex

Question

I have the following function, written in C++11, that uses regular expressions from the standard regex module. As its name suggests, it determines whether the given string str is a number prefixed with the given prefix.

bool isPrefixedNumber(const std::string &prefix, const std::string &str) { std::regex re(prefix + "[[:digit:]]+")) return std::regex_match(str, re); }

The problem is that I want prefix to be taken literally, i.e. I want

isPrefixedNumber("t.st", "tXst123")

to return false. Is there a way of constructing such a regular expression without the need to manually escape all occurrences of special characters in prefix? In another words, how to prevent the interpretation of special characters when passing a std::string into std::regex?

Note: The above function is just a simple illustration of my question. I need to prevent interpretation of strings in a much larger regular expression. I do not need another way of re-writing the function without regular expressions.

Ulrich Eckhardt · Accepted Answer · 2014-12-03 19:30:14Z

1

str.find(prefix) == 0 and str.find_first_not_of("0123456789", prefix.size()) != std::string::npos are the two things you want.

Whenever you have matching requirements, consider that a regex is actually a program that is parsed and executed at runtime. Put this in contrast with writing a few lines of C++ code and then decide which is better, but don't aim for regexes from the start.

answered Dec 3, 2014 at 19:30

Ulrich Eckhardt

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5 Comments

Platinum Azure Over a year ago

Yes-- seems like this might be an X/Y problem.

s3rvac Over a year ago

I totally agree with you that this is a better approach for this particular example. However, I need to prevent interpretation of strings in a much larger regular expression. I just came up with this simple example to illustrate my question. I will update the question to include this fact.

Ulrich Eckhardt Over a year ago

Okay. Still, when you say "don't interpret", it means verbatim use and therefore simple string matching, for which you should at least consider simple searching. That said, check out stackoverflow.com/questions/1252992/…, it seems to answer your question.

s3rvac Over a year ago

I was kind of hoping there would be a built-in way of doing this so I don't have to do it manually. Anyway, thanks!

Ulrich Eckhardt Over a year ago

File a feature request for Boost, this is indeed something that is a reasonably common need and one that can easily be done wrong (e.g. by forgetting one of the many special characters).

Jerry Coffin · Accepted Answer · 2014-12-03 18:28:29Z

I'd probably just match the two pieces separately, something along this general line:

std::string::size_type pos; std::regex re(prefix + "[[:digit:]]+")); return ((pos = str.find(prefix)) != std::string::npos) && std::regex_match(str.substr(pos+prefix.size()), re));

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