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Regex Tutorial
The term Regex stands for Regular expression. The regex or regexp or regular expression is a sequence of different characters which describe the particular search pattern. It is also referred/called as a Rational expression.
It is mainly used for searching and manipulating text strings. In simple words, you can easily search the pattern and replace them with the matching pattern with the help of regular expression.
This concept or tool is used in almost all the programming or scripting languages such as PHP, C, C++, Java, Perl, JavaScript, Python, Ruby, and many others. It is also used in word processors such as word which helps users for searching the text in a document, and also used in various IDEs.
The pattern defined by the regular expression is applied to the given string or a text from left to right.
Regular Expression Characters
There are following different type of characters of a regular expression:
- Metacharacters
- Quantifier
- Groups and Ranges
- Escape Characters or character classes
Metacharacters
| Metacharacters | Description | Example |
|---|---|---|
| ^ | This character is used to match an expression to its right at the start of a string. | ^a is an expression match to the string which starts with ‘a‘ such as “aab”, “a9c”, “apr”, “aaaaab”, etc. |
| $ | The $sign is used to match an expression to its left at the end of a string. | r$ is an expression match to a string which ends with r such as “aaabr”, “ar”, “r”, “aannn9r”, etc. |
| . | This character is used to match any single character in a string except the line terminator, i.e. /n. | b.x is an expression that match strings such as “bax”, “b9x”, “bar”. |
| | | It is used to match a particular character or a group of characters on either side. If the character on the left side is matched, then the right side’s character is ignored. | A|b is an expression which gives various strings, but each string contains either a or b. |
| It is used to escape a special character after this sign in a string. | ||
| A | It is used to match the character ‘A’ in the string. | This expression matches those strings in which at least one-time A is present. Such strings are “Amcx”, “mnAr”, “mnopAx4”. |
| Ab | It is used to match the substring ‘ab’ in the string. | This expression matches those strings in which ‘Ab’ is present at least one time. Such strings are “Abcx”, “mnAb”, “mnopAbx4”. |
Quantifiers
The quantifiers are used in the regular expression for specifying the number of occurrences of a character.
| Characters | Description | Example |
|---|---|---|
| + | This character specifies an expression to its left for one or more times. | s+ is an expression which gives “s”, “ss”, “sss“, and so on. |
| ? | This character specifies an expression to its left for 0 (Zero) or 1 (one)times. | aS? is an expression which gives either “a” or “as”, but not “ass”. |
| * | This character specifies an expression to its left for 0 or more times | Br* is an expression which gives “B”, “Br”, “Brr”, “Brrr”, and so on… |
| {x} | It specifies an expression to its left for only x times. | Mab{5} is an expression which gives the following string which contains 5 b’s: “Mabbbbb” |
| {x, } | It specifies an expression to its left for x or more times. | Xb{3, } is an expression which gives various strings containing at least 3 b’s. Such strings are “Xbbb”, “Xbbbb”, and so on. |
| {x,y} | It specifies an expression to its left, at least x times but less than y times. | Pr{3,6}a is an expression which provides two strings. Both strings are as follows: “Prrrr” and “Prrrrr” |
Groups and Ranges
The groups and ranges in the regular expression define the collection of characters enclosed in the brackets.
| Characters | Description | Example |
|---|---|---|
| ( ) | It is used to match everything which is in the simple bracket. | A(xy) is an expression which matches with the following string: “Axy” |
| { } | It is used to match a particular number of occurrences defined in the curly bracket for its left string. | xz{4,6} is an expression which matches with the following string: “xzzzzz” |
| [ ] | It is used to match any character from a range of characters defined in the square bracket. | xz[atp]r is an expression which matches with the following strings: “xzar”, “xztr”, and “xzpr” |
| [pqr] | It matches p, q, or r individually. | Following strings are matched with this expression: “p”, “q”, and “r”. |
| [pqr][xy] | It matches p, q, or r, followed by either x or y. | Following strings are matched with this expression: “px”, “qx”, and “rx”, “py”, “qy”, and “ry”. |
| (?: …) | It is used for matching a non-capturing group. | A(?:nt|pple) is an expression which matches to the following string: “Apple” |
| [^…..] | It matches a character which is not defined in the square bracket. | Suppose, Ab[^pqr] is an expression which matches only the following string: “Ab” |
| [a-z] | It matches letters of a small case from a to z. | This expression matches the strings such as: “a”, “python”, “good”. |
| [A-Z] | It matches letters of an upper case from A to Z. | This expression matches the strings such as: “EXCELLENT”, “NATURE”. |
| ^[a-zA-Z] | It is used to match the string, which is either starts with a small case or upper-case letter. | This expression matches the strings such as: “A854xb”, “pv4fv”, “cdux”. |
| [0-9] | It matches a digit from 0 to 9. | This expression matches the strings such as: “9845”, “54455” |
| [aeiou] | This square bracket only matches the small case vowels. | – |
| [AEIOU] | This square bracket only matches the upper-case vowels. | – |
| ab[^4-9] | It matches those digits or characters which are not defined in the square bracket. | This expression matches those strings which do not contain 5, 6, 7, and 8. |
Escape Characters or Character Classes
| Characters | Description |
|---|---|
| s | It is used to match a one white space character. |
| S | It is used to match one non-white space character. |