Regular expression matching

From Rosetta Code

The goal of this task is

• to match a string against a regular expression
• to substitute part of a string using a regular expression

Contents 1 AppleScript 2 C++ 3 C# 4 D 5 Haskell 6 J 7 Java 8 JavaScript 9 OCaml 10 Perl 11 PHP 12 Python 13 Raven 14 Ruby 15 Tcl 16 Toka

[edit] AppleScript

Library: Satimage.osax

try
    find text ".*string$" in "I am a string" with regexp
on error message
    return message
end try

try
    change "original" into "modified" in "I am the original string" with regexp
on error message
    return message
end try

[edit] C++

Works with: g++ version 4.0.2

Library: Boost

 #include <iostream>
 #include <string>
 #include <iterator>
 #include <boost/regex.hpp>
 
 int main()
 {
   boost::regex re(".* string$");
   std::string s = "Hi, I am a string";
 
   // match the complete string
   if (boost::regex_match(s, re))
     std::cout << "The string matches.\n";
   else
     std::cout << "Oops - not found?\n";
 
   // match a substring
   boost::regex re2(" a.*a");
   boost::smatch match;
   if (boost::regex_search(s, match, re2))
   {
     std::cout << "Matched " << match.length()
               << " characters starting at " << match.position() << ".\n";
     std::cout << "Matched character sequence: \""
               << match.str() << "\"\n";
   }
   else
   {
     std::cout << "Oops - not found?\n";
   }
 
   // replace a substring
   std::string dest_string;
   boost::regex_replace(std::back_inserter(dest_string),
                        s.begin(), s.end(),
                        re2,
                        "'m now a changed");
   std::cout << dest_string << std::endl;
 }

[edit] C#

Works with: .NET version 2.0+

Import System.Text.RegularExpressions;

string str = "I am a clever string";
string pattern = ".*clever.*";
Regex regex = new Regex(pattern);
if (  regex.IsMatch( str) ) {
    Console.WriteLine( "The string contains clever" );
}
if ( Regex.IsMatch( str, pattern ) ) {
    Console.WriteLine( "A more clever way to detect that the string contains clever" );
}

[edit] D

 import std.stdio, std.regexp;

 void main() {
     string s = "I am a string";

     // Test:
     if (search(s, r"string$"))
         writefln("Ends with 'string'");

     // Test, storing the regular expression:
     auto re1 = RegExp(r"string$");
     if (re1.search(s).test)
         writefln("Ends with 'string'");

     // Substitute:
     writefln(sub(s, " a ", " another "));

     // Substitute, storing the regular expression:
     auto re2 = RegExp(" a ");
     writefln(re2.replace(s, " another "));
 }

Note that in std.string there are string functions to perform those string operations in a faster way.

[edit] Haskell

Test

import Text.Regex

str = "I am a string"

case matchRegex (mkRegex ".*string$") str of
  Just _  -> putStrLn $ "ends with 'string'"
  Nothing -> return ()

Substitute

import Text.Regex

orig = "I am the original string"
result = subRegex (mkRegex "original") orig "modified
putStrLn $ result

[edit] J

J's regex support is built on top of PCRE.

   load'regex'               NB.  Load regex library
   str =: 'I am a string'   NB.  String used in examples.

Matching:

   '.*string$' rxeq str     NB.  1 is true, 0 is false
1

Substitution:

   ('am';'am still') rxrplc str
I am still a string

[edit] Java

Works with: Java version 1.5+

Test

String str = "I am a string";
if (str.matches(".*string$")) {
  System.out.println("ends with 'string'");
}

Substitute

String orig = "I am the original string";
String result = orig.replaceAll("original", "modified");
// result is now "I am the modified string"

[edit] JavaScript

Test/Match

     var subject = "Hello world!";
     
     // Two different ways to create the RegExp object
     // Both examples use the exact same pattern... matching "hello"
     var re_PatternToMatch = /Hello (World)/i; // creates a RegExp literal with case-insensitivity
     var re_PatternToMatch2 = new RegExp("Hello (World)", "i");
     
     // Test for a match - return a bool
     var isMatch = re_PatternToMatch.test(subject);
     
     // Get the match details
     //    Returns an array with the match's details
     //    matches[0] == "Hello world"
     //    matches[1] == "world"
     var matches = re_PatternToMatch2.exec(subject);

Substitute

     var subject = "Hello world!";
     
     // Perform a string replacement
     //    newSubject == "Replaced!"
     var newSubject = subject.replace(re_PatternToMatch, "Replaced");

[edit] OCaml

Test

#load "str.cma";;
let str = "I am a string";;
try
  ignore(Str.search_forward (Str.regexp ".*string$") str 0);
  print_endline "ends with 'string'"
with Not_found -> ()
;;

Substitute

#load "str.cma";;
let orig = "I am the original string";;
let result = Str.global_replace (Str.regexp "original") "modified" orig;;
(* result is now "I am the modified string" *)

[edit] Perl

Works with: Perl version 5.8.8

Test

$string = "I am a string";
if ($string =~ /string$/) {
  print "Ends with 'string'\n";
}

if ($string !~ /^You/) {
  print "Does not start with 'You'\n";
}

Substitute

$string = "I am a string";
$string =~ s/ a / another /; # makes "I am a string" into "I am another string"
print $string;

Test and Substitute

$string = "I am a string";
if ($string =~ s/\bam\b/was/) { # \b is a word border
  print "I was able to find and replace 'am' with 'was'\n";
}

Options

# add the following just after the last / for additional control
# g = globally (match as many as possible)
# i = case-insensitive
# s = treat all of $string as a single line (in case you have line breaks in the content)
# m = multi-line (the expression is run on each line individually)
 
$string =~ s/i/u/ig; # would change "I am a string" into "u am a strung"

[edit] PHP

Works with: PHP version 5.2.0

$string = 'I am a string';

Test

if (preg_match('/string$/', $string))
{
    echo "Ends with 'string'\n";
}

Replace

$string = preg_replace('/\ba\b/', 'another', $string);
echo "Found 'a' and replace it with 'another', resulting in this string: $string\n";

[edit] Python

Works with: Python version 2.5

Setup

import re
str = 'I am a string'

Test

if re.search(r'string$', str):
    print "Ends with 'string'"

Test, storing the compiled regular expression in a variable

regex = re.compile(r'string$')
if regex.search(str):
    print "Ends with 'string'"

To find all matches rather than just the first match, use re.findall rather than re.search.

Substitute

str = re.sub(r' a ', ' another ', str)

All instances of the specified pattern are replaced. To limit the number of instances replaced, specify the fourth argument to sub, the maximum number of replacements. To make a case-insensitive replacement, place (?i) at the beginning of the regular expression.

Substitute, storing the compiled regular expression in a variable

regex = re.compile(r' a ')
str = regex.sub(' another ', str)

Note: re.match() and regex.match() imply a "^" at the beginning of the regular expression. re.search() and regex.search() do not.

[edit] Raven

'i am a string' as str

Match:

str m/string$/
if  "Ends with 'string'\n" print

Replace:

str r/ a / another / print

[edit] Ruby

Test

 string="I am a string"
 puts "Ends with 'string'" if string[/string$/]
 puts "Does not start with 'You'" if !string[/^You/]

Substitute

 puts string.gsub(/ a /,' another ')
 #or
 string[/ a /]='another'
 puts string

Substitute using block

 puts(string.gsub(/\bam\b/) do |match|
        puts "I found #{match}"
        #place "was" instead of the match
        "was"
      end)

[edit] Tcl

Test

set theString "I am a string"
if {[regexp -- {string$} $theString]} {
  puts "Ends with 'string'\n"
}

if (![regexp -- {^You} $theString]) {
  puts "Does not start with 'You'\n"
}

Substitute

set theString = "I am a string"
puts [regsub -- { a } {I am a string} { another }]

[edit] Toka

Toka's regular expression library allows for matching, but does not yet provide for replacing elements within strings.

#! Include the regex library
needs regex

#! The two test strings
" This is a string" is-data test.1
" Another string" is-data test.2

#! Create a new regex named 'expression' which tries
#! to match strings beginning with 'This'.
" ^This" regex: expression

#! An array to store the results of the match 
#! (Element 0 = starting offset, Element 1 = ending offset of match)
2 cells is-array match

#! Try both test strings against the expression. 
#! try-regex will return a flag.  -1 is TRUE, 0 is FALSE
expression test.1 2 match try-regex .
expression test.2 2 match try-regex .