CODEWITHSUNDEEP
rubyregexstring
Daniel Beardsley
Daniel Beardsley

Reputation: 20367

Return first match of Ruby regex

I'm looking for a way to perform a regex match on a string in Ruby and have it short-circuit on the first match.

The string I'm processing is long and from what it looks like the standard way (match method) would process the whole thing, collect each match, and return a MatchData object containing all matches.

match = string.match(/regex/)[0].to_s

Upvotes: 114

Views: 92167

Answers (5)

Presidenten
Presidenten

Reputation: 6437

You could try String#[] (as in variableName[/regular expression/]).

This is an example output from IRB:

names = "erik kalle johan anders erik kalle johan anders"
# => "erik kalle johan anders erik kalle johan anders"
names[/kalle/]
# => "kalle"

Upvotes: 159

Benjamin Crouzier
Benjamin Crouzier

Reputation: 41895

You can use []: (which is like match)

"[email protected]"[/\+([^@]+)/, 1] # matches capture group 1, i.e. what is inside ()
# => "account2"
"[email protected]"[/\+([^@]+)/]    # matches capture group 0, i.e. the whole match
# => "+account2"

Upvotes: 89

Felix
Felix

Reputation: 4716

I am not yet sure whether this feature is awesome or just totally crazy, but your regex can define local variables.

/\$(?<dollars>\d+)\.(?<cents>\d+)/ =~ "$3.67" #=> 0
dollars #=> "3"

(Taken from http://ruby-doc.org/core-2.1.1/Regexp.html).

Upvotes: 13

Litmus
Litmus

Reputation: 10986

A Regular Expression (regex) is nothing but a finite state machine (FSM).

An FSM attempts to answer the question "Is this state possible or not?"

It keeps attempting to make a pattern match until a match is found (success), or until all paths are explored and no match was found (failure).

On success, the question "Is this state possible or not?" has been answered with a "yes". Hence no further matching is necessary and the regex returns.

See this and this for more on this.

Further: here is an interesting example to demonstrate how regex works. Here, a regex is used to detect if a give number is prime. This example is in perl, but it can as well be written in ruby.

Upvotes: 2

Slartibartfast
Slartibartfast

Reputation: 8805

If only an existence of a match is important, you can go with 

/regexp/ =~ "string"

Either way, match should only return the first hit, while scan searches throughout entire string. Therefore if 

matchData = "string string".match(/string/)
matchData[0]    # => "string"
matchData[1]    # => nil - it's the first capture group not a second match

Upvotes: 26

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