In PHP regular expressions, use ^ and $ anchors to match the beginning and end of a string respectively. 1. ^ means the beginning of the string, ensuring that the matching content appears from the beginning, such as /^hello/, verify whether it starts with hello; 2. $ means the end of the string, such as /.jpg$/, verify whether it ends with .jpg; 3. Use ^ and $ in combination to achieve a complete match, such as /^abc\d $/ to ensure that the entire string conforms to the specified format; 4. In multi-line mode, ^ and $ will match the beginning and end of each line respectively; 5. Note that the ending line breaks may affect the matching result, and you can use \s* or trim() to avoid problems. Mastering these details can improve the accuracy of regular expressions.

In PHP regular expressions, if you want to match a pattern that must appear at the beginning or end of a string, you need to use the "first line" and "first line" anchors. These anchors themselves do not match any characters, but rather indicate positions. Understanding how they are used will allow you to write more accurate regular expressions.

^ indicates the beginning of the string

This symbol is used to ensure that the matching content appears from the beginning of the string. For example, if you want to determine whether a string starts with "hello":

 if (preg_match('/^hello/', $string)) {
    // Match successfully}

As long as “hello” is not the first word to appear, it will not match.

Note: If you enable multi-line mode (add the m modifier), ^ will also match the beginning of each line.

Common uses:

• Verify that the input starts with a specific content
• Extract data with fixed format prefixes

$ means the end of the string

Similar to ^ , $ is another anchor point that matches the end position of the string. For example, you want to confirm whether a string ends in .jpg :

 if (preg_match('/\.jpg$/', $filename)) {
    // It's a picture file}

If the file name is photo.jpg.zip , it does not match; if it is photo.jpg , it does not match.

Similarly, in multi-line mode, $ will also match the end of each line.

Combination: Completely match the entire string

When you need to exactly match a string, you can use ^ and $ to limit the entire structure:

 if (preg_match('/^abc\d $/', $input)) {
    // The input must be abc followed by one or more numbers}

This can prevent partial matching situations like xabc123y . This writing method is very useful when verifying user input.

To give a few common examples:

• Email format verification (although it cannot be completely based on regularity)
• Fixed length number, password format requirements
• URL path rule matching (such as /user/\d )

Note: End line breaks may also affect the results

Sometimes when you are working on text, the ending may have a newline character \n or \r\n . If you use the $ anchor point but do not add the modifier D , it will still think that $ is preceded by the end of the string, not the newline.

One solution is to add \s* to the end to ignore the blank, or use trim() to process the string before making a match.

Basically that's it. Mastering the use of ^ and $ can help you avoid many mismatch problems. Although it may seem simple, details are easily overlooked in practical applications, especially when dealing with user input or complex formats.

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