Detailed analysis of matching single characters using regular expressions

This time I will bring you a detailed analysis of using regular expressions to match a single character. What are the precautions for using regular expressions to match a single character? The following is a practical case, let’s take a look.

The example in this article describes the regular expressiontutorial on matching a single character. Share it with everyone for your reference, as follows:

Note: In all examples, the regular expression matching results contain [ and ]## in the source text. #, some examples will be implemented using Java. If it is the usage of regular expressions in Java itself, it will be explained in the corresponding place. All java examples are tested under JDK1.6.0_13.

/**
 * 根据正则表达式和要匹配的源文本，输出匹配结果
 * @param regex 正则表达式
 * @param sourceText 要匹配的源文本
 */
public static void matchAndPrint(String regex, String sourceText){
  Pattern pattern = Pattern.compile(regex);
  Matcher matcher = pattern.matcher(sourceText);
  while(matcher.find()){
    System.out.println(matcher.group());
  }
}

1. Match plain text

1. Only one matching result

Yesterday is history, tomorrow is a mystery, but today is a gift.

today

[today] is a gift.

2. There are multiple matching results

Yesterday is history, tomorrow is a mystery, but today is a gift.

is

[today] is a gift.

is

##is

is

The regular expression is Letters are case-sensitive, but many regular expression implementations also support case-insensitive matching operations. In JavaScript

In java, if you want to be case-insensitive, when compiling the regular expression, you can specify:

Patternpattern = Pattern.compile(regex, Pattern.CASE_INSENSITIVE);

The regular expressions we saw earlier are all static plain text, and they do not reflect the power of regular expressions at all. Next, let's see how to use regular expressions to match unpredictable characters.

Special characters

Text:

orders1.txt

##orders2.txt

sales1.txt

salesA.txt

##orders3.txt

sales2.txt

sales.txt

sales.

orders1.txt

【sales1】.txt

【salesA】.txt

orders3.txt

【 sales2】.txt

【sales.】txt

Analysis: The regular expression sales. will combine stringsales and The file name composed of another note is found. From the results, it can be seen that. can match letters, numbers, and itself. 4 out of 7 files match this pattern.

If the matchAndPrint method is called, the output result is:

sales1

salesA

sales2

sales.

3. Match special characters

. Characters have special properties in regular expressions meaning. If you need a . in the pattern, you have to find a way to tell the regular expression that you need the . character itself rather than its special meaning in the regular expression. To do this, the . must be escaped by preceding it with a \ character. \ is also a metacharacter (metacharacter, indicating that this character has a special meaning, not the character meaning itself). Consider the following example.

Find files starting with na or sa, no matter what number follows it.

Text:

sales.txt

na1.txt

na2.txt

sa1.txt

sanatxt.txt

Regular expression: .a.. txt

Result:

[sal]es.txt

[na1].txt

【na2】.txt

【sa1】.txt

【sanatxt】.txt

Analysis: This regular rule found na1.txt, na2.txt, and sa1.txt, but also found two unexpected results. Because the . character in the regex .a..txt will match any character. To match the . character itself, you need to use \ escape. Modifying the regular expression to .a.\.txt can meet our needs.

Note: If you use java, then the regular expression .a.\.txt should be written as .a.\\.txt, because \ is also an escape character in the java language.

4. Summary

Regular expressions are often referred to as patterns. They are actually strings composed of some characters. These characters can be ordinary characters (plain text) or metacharacters (special characters with special meanings). Here is an introduction to how to use ordinary characters and metacharacters to match unit characters. .can match any character. \ is used to escape characters. In regular expressions, character sequences with special meanings always begin with the \ character.

I believe you have mastered the method after reading the case in this article. For more exciting information, please pay attention to other related articles on the php Chinese website!

Recommended reading:

Using php and js to implement regular password matching of numbers and letters

Regular in JQ Verification cannot contain Chinese methods

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