正则表达式仅在文本包含以下内容时匹配

Question

Given the following text

KEYWORD This is a test
We want to match the following groups 1:YES 2:YES 3:YES

I want to match with " 1:YES ", " 2:YES " and " 3:YES " using

((\d):YES)

If and only if the first word in the complete text is " KEYWORD "

Given this test:

This is a test
We want to match the following groups 1:YES 2:YES 3:YES

No matches should be found

Answer 1

Java (as with most regex engines) doesn't support unbounded length look behinds, however there is a work-around!

String str = "KEYWORD This is a test\n" +
        "We want to match the following groups 1:YES 2:YES 3:YES";
Matcher matcher = Pattern.compile("(?s)(?<=\\AKEYWORD\\b.{1,99999})(\\d+:YES)").matcher(str);
while (matcher.find()) {
    System.out.println(matcher.group(1));
}

Which outputs:

1:YES
2:YES
3:YES

The trick here is the look behind (?<=\\\\AKEYWORD.{1,99999}) which has a large (but not unbounded) length. (?s) means DOTALL flag (dot matches newline too) and \\A means start of input which is needed because ^ matches start of line when DOTALL flag is used.

Answer 2

Without tricking lookbhinds in Java you can capture \\d+:YES\\b strings with using \\G . \\G causes a match to start from where previous match ended or it will match beginning of string the same as \\A .

We are in need of its first capability:

(?:\AKEYWORD|\G(?!\A))[\s\S]*?(\d:YES\b)

Breakdown:

• (?: Start of non-capturing group
  • \\A Match beginning of subject string
  • KEYWORD Match keyword
  • | Or
  • \\G(?!\\A) Continue from where previous match ends
• ) End of NCG
• [\\s\\S]*? Match anything else un-greedily
• (\\d+:YES\\b) Match and capture our desired part

Live demo

Java code:

Pattern p = Pattern.compile("(?:\\AKEYWORD|\\G(?!\\A))[\\s\\S]*?(\\d+:YES\\b)");
Matcher m = p.matcher(string);                                   
while (m.find()) {
    System.out.println(m.group(1));
}

Live demo