Regex to partition multiline string

Question

Consider a multiline string consisting of N lines, like the following:

Line 1 text
Line 2 text
Line 3 text
...
Line n-1 text
Line n text
anchor=value
Line n+2 text
Line n+3 text
Line n+4 text
...
Line N text

The anchor key does not appear inside any of the lines and there may be spaces before the anchor, as well as around the = sign that follows it.

I need a regex that partitions the above string into 3 groups:

Line 1 to Line n (inclusive)
Anchor line (partition point)
Line n+2 to Line N (inclusive)

The closest I have got to the solution is

(?s)^(?:(?!anchor\s*=\s*).)+?\r|\nanchor\s*=\s*([^\r\n]+)(?:\r|\n)(.*)

but the above regex includes the entire text in the first matching group and populates the remaining 2 groups as expected.

An additional requirement is that the regex has to be as fast as possible, since it will be applied to large amounts of data. Note also that processing via a single regex is the only option in this use case.

Any ideas?

score 2 · Accepted Answer · answered Mar 02 '15 at 22:28

2

What about this regex?

(?s)^(.*?)(anchor\s*\=\s*[^\r\n]+)(.*?)

Or, to match the end of string,

(?s)^(.*?)(anchor\s*\=\s*[^\r\n]+)(.*?)$?

answered Mar 02 '15 at 22:28

Wiktor Stribiżew

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The first works if the non-greedy operator in the last group becomes greedy. The seconds works as is. Thanks! – PNS Mar 02 '15 at 22:40
@PNS, you are welcome. I knew the `(.*?)` pattern might need some boundary to hit, so I added the second option. – Wiktor Stribiżew Mar 02 '15 at 22:49
Yup, they seem to be fine. Thanks. – PNS Mar 02 '15 at 22:59

score 1 · Answer 2 · answered Mar 02 '15 at 22:18

1

If you need speed huge strings and regex is not the way to go. You have to have the entire string in memory to be able to use regex to tokenize it. Use Reader / InputStreams instead would be my recommendation.

answered Mar 02 '15 at 22:18

RudolphEst

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Sure, but in this use case the output comes from a library that only allows customization via a regular expression. – PNS Mar 02 '15 at 22:19
Now I am even more confused. If the output comes from a library, why are you doing the splitting? Do you mean the string is returned by a library? – RudolphEst Mar 02 '15 at 22:20
It is processed by a library that allows "injection" of regular expressions. – PNS Mar 02 '15 at 22:22
1

@PNS "library that only allows customization via a regular expression" this information should be placed in your question. Also describe how exactly this regex will be applied by your library: will it be argument for `split` or maybe for `find()`? – Pshemo Mar 02 '15 at 22:23
No split, it is applied to blocks of text like the one in the example, so we are probably looking for matches(). – PNS Mar 02 '15 at 22:34

score 1 · Answer 3 · answered Mar 02 '15 at 22:18

1

Well, you could first get the anchor, then split on it:

String anchor = str.replaceAll("(?ms).*?(anchor\\s*=.*?)$.*", "$1");
String lineParts = str.split("\\Q" + anchor + "\\E");

The "m" flag makes ^ and $ match start/end of lines.

answered Mar 02 '15 at 22:18

Bohemian

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Thanks but what is needed here is a single regex that does it all, because the code does not allow anything else. +1 anyway. :-) – PNS Mar 02 '15 at 22:29

Regex to partition multiline string

3 Answers3