以下は正規表現文字列の12の例である：

```
| Regex String | What it does | Where to use it | Explanation |
| --- | --- | --- | --- |
| `^a` | Matches any string that starts with an "a" character. | Can be used in text validation or search. Include it in the code where you need to validate or search for text. | `^` is an anchor that matches the start of a string. `a` is a character set that matches the "a" character. |
| `\d{3}` | Matches any string that contains exactly 3 digits. | Can be used in text validation or search. Include it in the code where you need to validate or search for text. | `\d` is a shorthand character set that matches any digit (0-9). `{3}` is a quantifier that specifies exactly 3 occurrences of the preceding element (in this case, `\d`). |
| `[A-Z]` | Matches any string that contains an uppercase letter from A to Z. | Can be used in text validation or search. Include it in the code where you need to validate or search for text. | `[A-Z]` is a character set that matches any uppercase letter from A to Z (inclusive). |
| `[aeiou]` | Matches any string that contains a vowel (lowercase). | Can be used in text validation or search. Include it in the code where you need to validate or search for text. | `[aeiou]` is a character set that matches any lowercase vowel (a, e, i, o, u). |
| `[^0-9]` | Matches any string that does not contain digits from 0 to 9.| Can be used in text validation or search. Include it in the code where you need to validate or search for text.| `[^0-9]` is a negated character set which means it will match anything except characters between 0 and 9 inclusive|
|`\w+@\w+\.\w+`|Matches email addresses.|Can be used when validating email addresses.|`\w+` matches one or more word characters before and after '@' sign and before '.' sign; '@' and '.' signs match themselves; `\w+` after '.' sign matches one or more word characters|
|`\bcat\b`|Matches whole word 'cat'.|\b asserts position at word boundary; can be used when searching for whole words only.|`\bcat\b`: '\b' before 'cat' ensures there's no word character before 'c'; 'cat' matches itself; '\b' after 't' ensures there's no word character after 't'.|
|
| `^.{3}$` | Matches any string that is exactly 3 characters long. | Can be used in text validation or search. Include it in the code where you need to validate or search for text. | `^` is an anchor that matches the start of a string. `.` is a character set that matches any character except line breaks. `{3}` is a quantifier that specifies exactly 3 occurrences of the preceding element (in this case, `.`). `$` is an anchor that matches the end of a string. |
| `\b\w{4}\b` | Matches any word that is exactly 4 characters long. | Can be used in text validation or search. Include it in the code where you need to validate or search for text. | `\b` is a word boundary that matches the position between a word character and a non-word character. `\w` is a shorthand character set that matches any word character (alphanumeric and underscore). `{4}` is a quantifier that specifies exactly 4 occurrences of the preceding element (in this case, `\w`). |
| `[a-z]{2,}` | Matches any string that contains at least 2 lowercase letters from a to z.| Can be used in text validation or search. Include it in the code where you need to validate or search for text.| `[a-z]` is a character set which means it will match characters between 'a' and 'z' inclusive; `{2,}` means at least two occurrences|
|`\d{3}-\d{2}-\d{4}`|Matches social security numbers.|Can be used when validating social security numbers.|`\d{3}` matches three digits; '-' sign matches itself; `\d{2}` matches two digits; '-' sign matches itself; `\d{4}` matches four digits|
|`(https?://)?(www\.)?(\w+)(\.\w+)+`|Matches URLs.|Can be used when validating URLs.|`(https?://)?`: 'http' followed by optional 's', followed by '://' - all optional; `(www\.)?`: 'www.' - optional; `(\w+)`: one or more word characters; `(\.\w+)+`: '.' followed by one or more word characters - one or more times|
```
Here are 12 examples of regex strings:

| Regex String | What it does | Where to use it | Explanation |
| --- | --- | --- | --- |
| `^a` | Matches any string that starts with an "a" character. | Can be used in text validation or search. Include it in the code where you need to validate or search for text. | `^` is an anchor that matches the start of a string. `a` is a character set that matches the "a" character. |
| `\d{3}` | Matches any string that contains exactly 3 digits. | Can be used in text validation or search. Include it in the code where you need to validate or search for text. | `\d` is a shorthand character set that matches any digit (0-9). `{3}` is a quantifier that specifies exactly 3 occurrences of the preceding element (in this case, `\d`). |
| `[A-Z]` | Matches any string that contains an uppercase letter from A to Z. | Can be used in text validation or search. Include it in the code where you need to validate or search for text. | `[A-Z]` is a character set that matches any uppercase letter from A to Z (inclusive). |
| `[aeiou]` | Matches any string that contains a vowel (lowercase). | Can be used in text validation or search. Include it in the code where you need to validate or search for text. | `[aeiou]` is a character set that matches any lowercase vowel (a, e, i, o, u). |
| `[^0-9]` | Matches any string that does not contain digits from 0 to 9.| Can be used in text validation or search. Include it in the code where you need to validate or search for text.| `[^0-9]` is a negated character set which means it will match anything except characters between 0 and 9 inclusive|
|`\w+@\w+\.\w+`|Matches email addresses.|Can be used when validating email addresses.|`\w+` matches one or more word characters before and after '@' sign and before '.' sign; '@' and '.' signs match themselves; `\w+` after '.' sign matches one or more word characters|
|`\bcat\b`|Matches whole word 'cat'.|\b asserts position at word boundary; can be used when searching for whole words only.|`\bcat\b`: '\b' before 'cat' ensures there's no word character before 'c'; 'cat' matches itself; '\b' after 't' ensures there's no word character after 't'.|
|
| `^.{3}$` | Matches any string that is exactly 3 characters long. | Can be used in text validation or search. Include it in the code where you need to validate or search for text. | `^` is an anchor that matches the start of a string. `.` is a character set that matches any character except line breaks. `{3}` is a quantifier that specifies exactly 3 occurrences of the preceding element (in this case, `.`). `$` is an anchor that matches the end of a string. |
| `\b\w{4}\b` | Matches any word that is exactly 4 characters long. | Can be used in text validation or search. Include it in the code where you need to validate or search for text. | `\b` is a word boundary that matches the position between a word character and a non-word character. `\w` is a shorthand character set that matches any word character (alphanumeric and underscore). `{4}` is a quantifier that specifies exactly 4 occurrences of the preceding element (in this case, `\w`). |
| `[a-z]{2,}` | Matches any string that contains at least 2 lowercase letters from a to z.| Can be used in text validation or search. Include it in the code where you need to validate or search for text.| `[a-z]` is a character set which means it will match characters between 'a' and 'z' inclusive; `{2,}` means at least two occurrences|
|`\d{3}-\d{2}-\d{4}`|Matches social security numbers.|Can be used when validating social security numbers.|`\d{3}` matches three digits; '-' sign matches itself; `\d{2}` matches two digits; '-' sign matches itself; `\d{4}` matches four digits|
|`(https?://)?(www\.)?(\w+)(\.\w+)+`|Matches URLs.|Can be used when validating URLs.|`(https?://)?`: 'http' followed by optional 's', followed by '://' - all optional; `(www\.)?`: 'www.' - optional; `(\w+)`: one or more word characters; `(\.\w+)+`: '.' followed by one or more word characters - one or more times|