View Category
Regex
Matching

Check if a string matches a regular expression

Display "ok" if "Hello" matches /[A-Z][a-z]+/
php
if(ereg('[A-Za-z]+', 'Hello')) {
echo "ok";
}
if(preg_match('/[A-Za-z]+/', 'Hello')>0) {
echo "ok";
}
erlang
String = "Hello", Regexp = "[A-Z][a-z]+",
is_match(String, Regexp) andalso (begin io:format("ok~n"), true end).
case re:run("Hello", "[A-Z][a-z]+") of {match, _} -> ok end.
cpp
if ((gcnew Regex("[A-Z][a-z]+"))->IsMatch("Hello")) Console::WriteLine("ok");
if (Regex::IsMatch("Hello", "[A-Z][a-z]+")) Console::WriteLine("ok");
Regex^ rx = gcnew Regex("[A-Z][a-z]+");
if (rx->IsMatch("Hello")) Console::WriteLine("ok");
cmatch what;
if (regex_match("Hello", what, regex("[A-Z][a-z]+")))
cout << "ok" << endl;

Check if a string matches with groups

Display "two" if "one two three" matches /one (.*) three/
php
preg_match('/one (.*) three/', 'one two three', $matches);
echo $matches[1];
ereg('one (.*) three', 'one two three', $regs);
echo $regs[1];
erlang
case re:run("one two three", "one (.*) three", [{capture, [1], list}]) of {match, Res} -> hd(Res) end.
cpp
Match^ match = Regex::Match("one two three", "one (.*) three");
if (match->Success) Console::WriteLine("{0}", match->Groups[1]->Captures[0]);
cmatch what;
if (regex_match("one two three", what, regex("one (.*) three")))
cout << what[1] << endl;
Searching

Check if a string contains a match to a regular expression

Display "ok" if "abc 123 @#$" matches /\d+/
php
if (preg_match("/\d+/", "abc 123 @#$"))
echo "ok";
erlang
% Erlang uses 'egrep'-compatible regular expressions, so shortcuts like '\d' not supported
String = "abc 123 @#$", Regexp = "[0-9]+",
is_match(String, Regexp) andalso (begin io:format("ok~n"), true end).
case re:run("abc 123 @#$", "\\d+") of {match, _} -> ok end.
cpp
if (Regex::IsMatch("abc 123 @#$", "\\d+")) Console::WriteLine("ok");

Loop through a string matching a regex and performing an action for each match

Create a list [fish1,cow3,boat4] when matching "(fish):1 sausage (cow):3 tree (boat):4" with regex /\((\w+)\):(\d+)/
php
preg_match_all("/\((\w+)\):(\d+)/", "(fish):1 sausage (cow):3 tree (boat):4", $matches);
for ($i=0, $c=count($matches[0]); $i < $c; $i++) {
$list[] = $matches[1][$i].$matches[2][$i];
}
erlang
solve(S) ->
R = "\\((\\w+?)\\):(\\d+)",
{match, M} = re:run(S,R, [global, {capture, all_but_first, list}]),
[ A++N || [A, N] <- M].
cpp
Match^ match = Regex::Match("(fish):1 sausage (cow):3 tree (boat):4", "\\((\\w+)\\):(\\d+)");

while (match->Success)
{
list->Add(match->Groups[1]->Captures[0]->ToString() + match->Groups[2]->Captures[0]->ToString());
match = match->NextMatch();
}
Replacing

Replace the first regex match in a string with a static string

Transform "Red Green Blue" into "R*d Green Blue" by replacing /e/ with "*"
php
echo preg_replace('/e/', '*', "Red Green Blue", 1);
erlang
{ok, Replaced, _} = regexp:sub("Red Green Blue", "e", "*"),
re:replace("Red Green Blue", "e", "*", [{return, list}]).
cpp
String^ Replaced = (gcnew Regex("e"))->Replace("Red Green Blue", "*", 1);

Replace all regex matches in a string with a static string

Transform "She sells sea shells" into "She X X shells" by replacing /se\w+/ with "X"
php
echo preg_replace('/se\w+/', 'X', 'She sells sea shells');
erlang
% Erlang uses 'egrep'-compatible regular expressions, so shortcuts like '\w' not supported
{ok, Replaced, _} = regexp:gsub("She sells sea shells", "se[A-Za-z0-9_]+", "X"),
re:replace("She sells sea shells", "se\\w+", "X", [global, {return, list}]).
cpp
String^ Replaced = (gcnew Regex("se\\w+"))->Replace("She sells sea shells", "X");
String^ Replaced = Regex::Replace("She sells sea shells", "se\\w+", "X");

Replace all regex matches in a string with a dynamic string

Transform "The {Quick} Brown {Fox}" into "The kciuQ Brown xoF" by reversing words in braces using the regex /\{(\w+)\}/.
php
// We have to use the e-modifier
preg_replace("/\{(\w+)\}/e", "''.strrev('\\1').''", "The {Quick} Brown {Fox}");
erlang
% Erlang regular expressions lack both group capture and backreferences, thus this problem is not directly
% solvable. Presented solution is close, but not on-spec

String = "The {Quick} Brown {Fox}",
{match, FieldList} = regexp:matches(String, "\{([A-Za-z0-9_]+)\}"),

NewString = lists:foldl(fun ({Start, Length}, S) -> replstr(S, lists:reverse(string:substr(S, Start, Length)), Start) end, String, FieldList),
cpp
String^ Replaced = (gcnew Regex("{(\\w+)}"))->Replace("The {Quick} Brown {Fox}", gcnew MatchEvaluator(&RegRep::RepGroup));
String^ Replaced = Regex::Replace("The {Quick} Brown {Fox}", "{(\\w+)}", gcnew MatchEvaluator(&RegRep::RepGroup));