Need a Regex that extracts a string between two "delimiting" strings

Question

I need to get the string between by_ and _on.
So far I have this, but don't understand how to truncate the actual "string delimiters":
by_(.*)_on

Sample input:
Files_by_wesasegeaazedude_on_January_26.jpg
Current Match:
by_wesasegeaazedude_on
Needed Match:
wesasegeaazedude

See [*Java Regex Capturing Groups*](http://stackoverflow.com/questions/17969436/java-regex-capturing-groups) — Wiktor Stribiżew, Apr 15 '16 at 16:47
usually its the first index(not 0) that contains the first captured group — rock321987, Apr 15 '16 at 16:47

score 1 · Answer 1 · edited Apr 15 '16 at 19:46

1

You can simply use positive lookarounds:

String regex = "(?<=by_).*(?=_on)";

What this regex does is:

match anything: .*
that is preceded by by_: (?<=by_)
and followed by _on: (?=_on)

edited Apr 15 '16 at 19:46

Wiktor Stribiżew

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answered Apr 15 '16 at 16:48

Maljam

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Why do you suggest an inefficient pattern? OP has got the same pattern that is way more efficient. – Wiktor Stribiżew Apr 15 '16 at 16:48
@WiktorStribiżew because he is saying that he doesn't want to match the `by_` and `_on` – Maljam Apr 15 '16 at 16:50
@Maljam thanks! it worked – JESTech Apr 15 '16 at 17:45
Worked or not, _Anytime you lead a regular expression by an assertion, you instantly add %200 more overhead._ This can increase by as much as %10-20,000 depending on what follows it. – Apr 15 '16 at 18:02
@sln so, which expression do you propose? It's good you know all that technical stuff but a quick helping hand is better appreciated most of the time. – JESTech Apr 15 '16 at 18:32
@DevJE - You choose ([Benchmark app](http://www.regexformat.com/scrn7/s7_b4.jpg)). _Regex1: (?<=by_).*(?=_on) Elapsed Time: 2.46 s, 2460.14 ms, 2460140 µs Regex2: by_(.*)_on Elapsed Time: 0.39 s, 391.92 ms, 391916 µs_ – Apr 15 '16 at 18:50
@sln you are right, it is more efficient technically, but you are using an example that intentionally chooses a really bad scenario, but if you are using it for file paths, or other small strings, the difference if negligible.. – Maljam Apr 15 '16 at 21:27

score 1 · Accepted Answer · answered Apr 15 '16 at 16:52

Your expression is good^*. All you need to do is extracting the content of the first capturing group:

Pattern regex = Pattern.compile("by_(.*)_on");
String str = "Files_by_wesasegeaazedude_on_January_26.jpg";
Matcher m = regex.matcher(str);
if (m.find()) {
    String res = m.group(1);
}

Demo.

^* Well, almost good. If you expect inputs with multiple file names on the same line, you may want to consider using reluctant qualifier, i.e. by_(.*?)_on

Andy Turner · Answer 3 · 2016-04-15T17:06:34.297

1

I would do this without regular expressions.

int start = str.indexOf("by_");
int end = str.indexOf("_on", start + 1); // or lastIndexOf("_on"), for greedy match.
assert start > 0 && end > start;
String part = str.substring(start + 3, end);

edited Apr 15 '16 at 17:06

answered Apr 15 '16 at 16:54

Andy Turner

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Plus one for a person who do not try to make everything by regex ) – Alex Salauyou Apr 15 '16 at 16:57
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@Sasha I actually misread the question; but I'd still do it without regex :) – Andy Turner Apr 15 '16 at 17:08

Need a Regex that extracts a string between two "delimiting" strings

3 Answers3