CppStdRegex.md

(C++) std::regex

std::regex is an STL class for regular expressions.

The default regular expression notation is that of ECMAScript [1], but the regex can use POSIX, awk, grep and egrep notation additionally [2].

ECMAScript table

Adapted from [1]:

Text	description
[[:alnum:]]	alpha-numerical character
[[:alpha:]]	alphabetic character
[[:blank:]]	blank character
[[:cntrl:]]	control character
[[:digit:]]	decimal digit character
[[:graph:]]	character with graphical representation
[[:lower:]]	lowercase letter
[[:print:]]	printable character
[[:punct:]]	punctuation mark character
[[:space:]]	whitespace character
[[:upper:]]	uppercase letter
[[:xdigit:]]	hexadecimal digit character
[[:d:]]	decimal digit character
[[:w:]]	word character
[[:s:]]	whitespace character

Example: multipliers

The example show the use of:

• .: can be anything
• [[:digit:]]: a digit
• ?: zero or one repeats of the preceding thing
• +: one or more repeats of the preceding thing
• *: zero or more repeats of the preceding thing
• {2}: two repeats of the preceding thing

#include <cassert>
#include <regex>
#include <string>

int main()
{
  assert(!std::regex_match("", std::regex("."))); //One anything
  assert(!std::regex_match("", std::regex("[[:digit:]]"))); //One digit
  assert( std::regex_match("", std::regex("[[:digit:]]?"))); //Zero or one digit
  assert(!std::regex_match("", std::regex("[[:digit:]]+"))); //One or more digits
  assert( std::regex_match("", std::regex("[[:digit:]]*"))); //Zero or more digits
  assert( std::regex_match("", std::regex("[[:digit:]]{0}"))); //Zero digits

  assert(std::regex_match("1", std::regex("."))); //One anything
  assert(std::regex_match("1", std::regex("[[:digit:]]"))); //One digit
  assert(std::regex_match("1", std::regex("[[:digit:]]?"))); //Zero or one digit
  assert(std::regex_match("1", std::regex("[[:digit:]]+"))); //One or more digits
  assert(std::regex_match("1", std::regex("[[:digit:]]*"))); //Zero or more digits
  assert(std::regex_match("1", std::regex("[[:digit:]]{1}"))); //One digit

  assert(!std::regex_match("12", std::regex("."))); //One anything
  assert(!std::regex_match("12", std::regex("[[:digit:]]"))) ; //One digit
  assert(!std::regex_match("12", std::regex("[[:digit:]]?"))); //Zero or one digit
  assert( std::regex_match("12", std::regex("[[:digit:]]+"))); //One or more digits
  assert( std::regex_match("12", std::regex("[[:digit:]]*"))); //Zero or more digits
  assert( std::regex_match("12", std::regex("[[:digit:]]{2}"))); //Two digits

}

Example: is_benelux_web_domain

The example show the use of:

• |: or
• (): group
• \\.: a literal dot, .. The backslash escapes the dot being a wildcard. Because the backslash is a std::string escape character itself, it needs to be escaped by anothed backslash

#include <cassert>
#include <regex>
#include <string>
//A (simplified) Benelux (Dutch, Flemisch, Luxembourg) URL:
//  - has one or more alphanumeric characters
//  - ends on '.nl', '.be' or '.lu'
int main()
{
  const std::regex benelux_url("[[:alnum:]]+\\.(nl|be|lu)");
  assert( std::regex_match("nu.nl", benelux_url));
  assert( std::regex_match("k3.be", benelux_url));
  assert( std::regex_match("start.lu", benelux_url));
  assert(!std::regex_match("lemonde.fr", benelux_url));
  assert(!std::regex_match("nlbelu", benelux_url));
}

Example programs and code snippets

• RegexTester: tool to test regular expressions
• is_dutch_postal_code: function to test for a Dutch postal code (e.g. 1234 AB)

External links

• std::regex homepage
• cplusplus.com's ECMA script page
• C++ Weekly, Episode 62: std::regex
• C++ Weekly, Epsiode 74: std::regex optimize

References

• [1] Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Page 1071, 37.6 'Advice', item 3: 'The default regular expression notation is that of ECMAScript'
• [2] Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Page 1071, 37.6 'Advice', item 9: 'regex can use ECMAScript, POSIX, awk, grep and egrep notation'