Write 12 examples of regex strings using proper syntax. Display in table format. In the first column, write the string. In the second column, write what it does. In the third column, write where you would use it, and where it should be included in your file. In the fourth column, break it down and explain each part in detail. 

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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'.|
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| `^.{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|
What are the forbidden symbols in regex strings?