Performance comparision between Substring vs RegEx when start index is known

Question

I need to extract the first integer found in a java.lang.String and am unsure as to whether to try and use a substring approach or a regex approach:

// Want to extract the 510 into an int.
String extract = "PowerFactor510";

// Either:
int num = Integer.valueof(extract.substring(???));

// Or a regex solution, something like:
String regex = "\\d+";
Matcher matcher = new Matcher(regex);
int num = matcher.find(extract);

So I ask:

Which type of solution is more appropriate here, and why?; and
If the substring approach is more appropriate, what could I use to indicate the beginning of a number?
Else, if the regex is the appropriate solution, what is the regex/pattern/matcher/method I should use to extract the number?

Note: The string will always begin with the word PowerFactor followed by a non-negative integer. Thanks in advance!

Is regex really faster than substring(11)? The first part is always fixed... I don't think that parsing a regex, going through the string and extracting the appropriate group would be quicker than to just chop off the first 11 chars... — ppeterka
– ppeterka, Commented Mar 25, 2013 at 13:26

Community · Accepted Answer · 2017-05-23 12:00:10Z

9

The string will always begin with the word "PowerFactor" followed by a non-negative integer

This means you know exactly at which index you will find the number, i would say you better use the substring directly, at least considering the performance it would be much faster than searching and matching work.

extract.substring("PowerFactor".length());

I could not find any direct comparision but you can read about each one of the two options:

edited May 23, 2017 at 12:00

CommunityBot

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answered Mar 25, 2013 at 13:26

CloudyMarble

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tmwanik · Accepted Answer · 2013-03-25 15:08:32Z

1

Was a bit curious and tried the following

String extract = "PowerFactor510";
long l = System.currentTimeMillis();
System.out.println(extract.replaceAll("\\D", ""));
System.out.println(System.currentTimeMillis() - l);

System.out.println();

l = System.currentTimeMillis();
System.out.println(extract.substring("PowerFactor".length()));
System.out.println(System.currentTimeMillis() - l);

And it tuned out that the second test was much faster, so substring wins.

edited Mar 25, 2013 at 15:08

answered Mar 25, 2013 at 13:44

tmwanik

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3 Comments

tchrist Over a year ago

Why in the world would you put \D in brackets there?

tmwanik Over a year ago

@tchrist Editted the answer

ingyhere Over a year ago

That is a horrible test. the replaceAll method of the String class performs an inline compile on the RegEx before processing it. The method does not yield a suitable test against Pattern/Matcher or anything RegEx related. Speed differences you are seeing are related to object creation and GC in the JVM.

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