Regular expressions
You are encouraged to solve this task according to the task description, using any language you may know.
The goal of this task is
- to match a string against a regular expression
- to substitute part of a string using a regular expression
AppleScript
try
find text ".*string$" in "I am a string" with regexp
on error message
return message
end try
try
change "original" into "modified" in "I am the original string" with regexp
on error message
return message
end try
ALGOL 68
The routines grep in strings and sub in string are not part of ALGOL 68's standard prelude.
<lang algol>INT match=0, no match=1, out of memory error=2, other error=3;
STRING str := "i am a string";
- Match: #
STRING m := "string$"; INT start, end; IF grep in string(m, str, start, end) = match THEN printf(($"Ends with """g""""l$, str[start:end])) FI;
- Replace: #
IF sub in string(" a ", " another ",str) = match THEN printf(($gl$, str)) FI;</lang> Output:
Ends with "string" i am another string
Standard ALGOL 68 does have an primordial form of pattern matching called a format. This is designed to extract values from input data. But it can also be used for outputting (and transputting) the original data.
For example:<lang algol> FORMAT pattern = $ddd" "c("cats","dogs")$; FILE file; STRING book; associate(file, book); on value error(file, (REF FILE f)BOOL: stop); on format error(file, (REF FILE f)BOOL: stop);
book := "100 dogs"; STRUCT(INT count, type) dalmatians;
getf(file, (pattern, dalmatians)); print(("Dalmatians: ", dalmatians, new line)); count OF dalmatians +:=1; printf(($"Gives: "$, pattern, dalmatians, $l$))</lang> Output:
Dalmatians: +100 +2 Gives 101 dogs
C
As far as I can see, POSIX defined function for regex matching, but nothing for substitution. So we must do all the hard work by hand. The complex-appearing code could be turned into a function.
<lang c>#include <stdio.h>
- include <stdlib.h>
- include <sys/types.h>
- include <regex.h>
- include <string.h>
int main() {
regex_t preg;
regmatch_t substmatch[1];
const char *tp = "string$";
const char *t1 = "this is a matching string";
const char *t2 = "this is not a matching string!";
const char *ss = "istyfied";
regcomp(&preg, "string$", REG_EXTENDED);
printf("'%s' %smatched with '%s'\n", t1,
(regexec(&preg, t1, 0, NULL, 0)==0) ? "" : "did not ", tp);
printf("'%s' %smatched with '%s'\n", t2,
(regexec(&preg, t2, 0, NULL, 0)==0) ? "" : "did not ", tp);
regfree(&preg);
/* change "a[a-z]+" into "istifyed"?*/
regcomp(&preg, "a[a-z]+", REG_EXTENDED);
if ( regexec(&preg, t1, 1, substmatch, 0) == 0 )
{
//fprintf(stderr, "%d, %d\n", substmatch[0].rm_so, substmatch[0].rm_eo);
char *ns = malloc(substmatch[0].rm_so + 1 + strlen(ss) +
(strlen(t1) - substmatch[0].rm_eo) + 2);
memcpy(ns, t1, substmatch[0].rm_so+1);
memcpy(&ns[substmatch[0].rm_so], ss, strlen(ss));
memcpy(&ns[substmatch[0].rm_so+strlen(ss)], &t1[substmatch[0].rm_eo],
strlen(&t1[substmatch[0].rm_eo]));
ns[ substmatch[0].rm_so + strlen(ss) +
strlen(&t1[substmatch[0].rm_eo]) ] = 0;
printf("mod string: '%s'\n", ns);
free(ns);
} else {
printf("the string '%s' is the same: no matching!\n", t1);
}
regfree(&preg);
return 0;
}</lang>
C++
#include <iostream>
#include <string>
#include <iterator>
#include <boost/regex.hpp>
int main()
{
boost::regex re(".* string$");
std::string s = "Hi, I am a string";
// match the complete string
if (boost::regex_match(s, re))
std::cout << "The string matches.\n";
else
std::cout << "Oops - not found?\n";
// match a substring
boost::regex re2(" a.*a");
boost::smatch match;
if (boost::regex_search(s, match, re2))
{
std::cout << "Matched " << match.length()
<< " characters starting at " << match.position() << ".\n";
std::cout << "Matched character sequence: \""
<< match.str() << "\"\n";
}
else
{
std::cout << "Oops - not found?\n";
}
// replace a substring
std::string dest_string;
boost::regex_replace(std::back_inserter(dest_string),
s.begin(), s.end(),
re2,
"'m now a changed");
std::cout << dest_string << std::endl;
}
C#
<lang csharp>using System.Text.RegularExpressions;
... </lang>
D
import std.stdio, std.regexp;
void main() {
string s = "I am a string";
// Test:
if (search(s, r"string$"))
writefln("Ends with 'string'");
// Test, storing the regular expression:
auto re1 = RegExp(r"string$");
if (re1.search(s).test)
writefln("Ends with 'string'");
// Substitute:
writefln(sub(s, " a ", " another "));
// Substitute, storing the regular expression:
auto re2 = RegExp(" a ");
writefln(re2.replace(s, " another "));
}
Note that in std.string there are string functions to perform those string operations in a faster way.
Forth
Test/Match
include ffl/rgx.fs \ Create a regular expression variable 'exp' in the dictionary rgx-create exp \ Compile an expression s" Hello (World)" exp rgx-compile [IF] .( Regular expression successful compiled.) cr [THEN] \ (Case sensitive) match a string with the expression s" Hello World" exp rgx-cmatch? [IF] .( String matches with the expression.) cr [ELSE] .( No match.) cr [THEN]
Haskell
Test
import Text.Regex str = "I am a string" case matchRegex (mkRegex ".*string$") 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
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$' rxeq str NB. 1 is true, 0 is false 1
Substitution:
('am';'am still') rxrplc str
I am still a string
Java
Test
String str = "I am a string";
if (str.matches(".*string$")) {
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"
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");
Objective-C
Test
<lang objc> NSString *str = @"I am a string"; NSString *regex = @".*string$";
NSPredicate *pred = [NSPredicate predicateWithFormat:@"SELF MATCHES %@", regex];
if ([pred evaluateWithObject:str]) {
NSLog(@"ends with 'string'");
} </lang> Unfortunately this method cannot find the location of the match or do substitution.
OCaml
Test
#load "str.cma";; let str = "I am a string";; try ignore(Str.search_forward (Str.regexp ".*string$") 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" *)
Perl
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"
PHP
$string = 'I am a string';
Test
if (preg_match('/string$/', $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: $string\n";
Python
<lang python>import re
string = "This is a string"
if re.search('string$',string):
print "Ends with string."
string = re.sub(" a "," another ",string) print string</lang>
Raven
'i am a string' as str
Match:
str m/string$/ if "Ends with 'string'\n" print
Replace:
str r/ a / another / print
Ruby
Test
string="I am a string" puts "Ends with 'string'" if string[/string$/] 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)
Smalltalk
<lang smalltalk>|re s s1| re := Regex fromString: '[a-z]+ing'. s := 'this is a matching string'. s1 := 'this does not match'.
(s =~ re) ifMatched: [ :b |
b match displayNl
]. (s1 =~ re) ifMatched: [ :b |
'Strangely matched!' displayNl
] ifNotMatched: [
'no match!' displayNl
].
(s replacingRegex: re with: 'modified') displayNl.</lang>
Tcl
Test
set theString "I am a string"
if {[regexp -- {string$} $theString]} {
puts "Ends with 'string'\n"
}
if (![regexp -- {^You} $theString]) {
puts "Does not start with 'You'\n"
}
Substitute
set theString = "I am a string"
puts [regsub -- { a } {I am a string} { another }]
Toka
Toka's regular expression library allows for matching, but does not yet provide for replacing elements within strings.
#! Include the regex library needs regex #! The two test strings " This is a string" is-data test.1 " Another string" is-data test.2 #! Create a new regex named 'expression' which tries #! to match strings beginning with 'This'. " ^This" regex: expression #! An array to store the results of the match #! (Element 0 = starting offset, Element 1 = ending offset of match) 2 cells is-array match #! Try both test strings against the expression. #! try-regex will return a flag. -1 is TRUE, 0 is FALSE expression test.1 2 match try-regex . expression test.2 2 match try-regex .
Vedit macro language
Vedit can perform searches and matching with either regular expressions, pattern matching codes or plain text. These examples use regular expressions.
Match text at cursor location: <lang vedit> if (Match(".* string$", REGEXP)==0) {
Statline_Message("This line ends with 'string'")
} </lang>
Search for a pattern: <lang vedit> if (Search("string$", REGEXP+NOERR)) {
Statline_Message("'string' at and of line found")
} </lang>
Replace: <lang vedit> Replace(" a ", " another ", REGEXP+NOERR) </lang>