正则表达式检查字符串是否匹配特定模式

Question

How to use Java regular expression to check if a string follows a certain pattern ? eg to check if a message first starts with " In the morning ", then followed by any words, then followed by " In the afternoon ", and then followed by any words.

I have tried to look up the regex syntax but found it difficult to understand.

I have tried to use | , but this is an "Or" operator. And it does not specify the ordering of first matching " In the morning " and then " In the afternoon ".

Pattern pattern = Pattern.compile("\\bIn the morning\\b|\\bIn the afternoon\\b");
Matcher matcher = pattern.matcher("In the morning I read the news, then I start my work. In the afternoon I have my lunch.");

Answer 1

^In the morning .*In the afternoon.*

The ^ matches the start of the expression to match. The .* matches zero or more non return characters.

You can also put parenthesis around the .* to form capture groups to find out what actually you did in the morning and afternoon

^In the morning (.*)In the afternoon (.*)

Answer 2

regex101

String str = "In the morning I read the news, then I start my work. In the afternoon I have my lunch.";
Pattern pattern = Pattern.compile("^\\bIn the morning\\b.*\\bIn the afternoon\\b.*$");
Matcher matcher = pattern.matcher(str);

if (matcher.matches())
    System.out.println("match");
else
    System.err.println("not match");

• ^ asserts position at the start of a line
• \\b assert position at a word boundary: (^\\w|\\w$|\\W\\w|\\w\\W)
• . matches any character (except for line terminators)
• * matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
• $ asserts position at the end of a line

Answer 3

I think you want .* in between the two phrases, rather than an alternation. Try this version:

Pattern pattern = Pattern.compile("\\bIn the morning\\b.*\\bIn the afternoon\\b");
Matcher matcher = pattern.matcher("In the morning I read the news, then I start my work. In the afternoon I have my lunch.");
if (matcher.find()) {
    System.out.println(matcher.group());
}

This prints:

In the morning I read the news, then I start my work. In the afternoon