Java regex to extract text sequences across multiple lines

Question

Given an excerpt of text like

Preface (optional, up to multiple lines)
Main : sequence1
   sequence2
   sequence3
   sequence4
Epilogue (optional, up to multiple lines)

which Java regular expression could be used to extract all the sequences (i.e. sequence1, sequence2, sequence3, sequence4 above)? For example, a Matcher.find() loop?

Each "sequence" is preceded by and may also contain 0 or more white spaces (including tabs).

The following regex

(?m).*Main(?:[ |t]+:(?:[ |t]+(\S+)[\r\n])+

only yields the first sequence (sequence1).

Does it mean you need to get multiple matches of the non-whitespace chunks that have some horizontal whitespaces on the subsequent lines after Main :? — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Dec 20, 2016 at 23:47
Use String p = "(?m)(?:\\G(?!\\A)[^\\S\r\n]+|^Main\\s*:\\s*)(\\S+)\r?\n?"; — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Dec 20, 2016 at 23:53

Wiktor Stribiżew · Accepted Answer · 2016-12-20 23:57:21Z

3

You may use the following regex:

(?m)(?:\G(?!\A)[^\S\r\n]+|^Main\s*:\s*)(\S+)\r?\n?

Details:

(?m) - multiline mode on
(?:\G(?!\A)[^\S\r\n]+|^Main\s*:\s*) - either of the two:
- \G(?!\A)[^\S\r\n]+ - end of the previous successful match (\G(?!\A)) and then 1+ horizontal whitespaces ([^\S\r\n]+, can be replaced with [\p{Zs}\t]+ or [\s&&[^\r\n]]+)
- | - or
- ^Main\s*:\s* - start of a line, Main, 0+ whitespaces, :, 0+ whitespaces
(\S+) - Group 1 capturing 1+ non-whitespace symbols
\r?\n? - an optional CR and an optional LF.

See the Java code below:

String p = "(?m)(?:\\G(?!\\A)[^\\S\r\n]+|^Main\\s*:\\s*)(\\S+)\r?\n?";
String s = "Preface (optional, up to multiple lines)...\nMain : sequence1\n   sequence2\n   sequence3\n   sequence4\nEpilogue (optional, up to multiple lines)";
Matcher m = Pattern.compile(p).matcher(s);
while(m.find()) {
    System.out.println(m.group(1));
}

answered Dec 20, 2016 at 23:57

Wiktor Stribiżew

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4 Comments

Wiktor Stribiżew Over a year ago

See a Java demo.

PNS Over a year ago

It works, thanks. Ideally, I would like something without anchors (\G or \A), but still it does the job. Maybe a simpler version exists. :-)

Wiktor Stribiżew Over a year ago

With 1 regex pass, this is the only way.

PNS Over a year ago

It is definitely an elegant one. If no simpler version appears, it will also become the selected answer. Thanks again. :-)

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