Translations:

What is Regular Expression?

A regular expression is a group of characters or symbols which is used to find a specific pattern in a text.

A regular expression is a pattern that is matched against a subject string from left to right. Regular expressions are used to replace text within a string, validating forms, extracting a substring from a string based on a pattern match, and so much more. The term "regular expression" is a mouthful, so you will usually find the term abbreviated to "regex" or "regexp".

Imagine you are writing an application and you want to set the rules for when a user chooses their username. We want to allow the username to contain letters, numbers, underscores and hyphens. We also want to limit the number of characters in the username so it does not look ugly. We can use the following regular expression to validate the username:

The regular expression above can accept the strings john_doe, jo-hn_doe and john12_as. It does not match Jo because that string contains an uppercase letter and also it is too short.

1. Basic Matchers

A regular expression is just a pattern of characters that we use to perform a search in a text. For example, the regular expression the means: the letter t, followed by the letter h, followed by the letter e.

"the" => The fat cat sat on the mat.

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The regular expression 123 matches the string 123. The regular expression is matched against an input string by comparing each character in the regular expression to each character in the input string, one after another. Regular expressions are normally case-sensitive so the regular expression The would not match the string the.

"The" => The fat cat sat on the mat.

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2. Meta Characters

Meta characters are the building blocks of regular expressions. Meta characters do not stand for themselves but instead are interpreted in some special way. Some meta characters have a special meaning and are written inside square brackets. The meta characters are as follows:

Meta character	Description
.	Period matches any single character except a line break.
[ ]	Character class. Matches any character contained between the square brackets.
[^ ]	Negated character class. Matches any character that is not contained between the square brackets
*	Matches 0 or more repetitions of the preceding symbol.
+	Matches 1 or more repetitions of the preceding symbol.
?	Makes the preceding symbol optional.
{n,m}	Braces. Matches at least "n" but not more than "m" repetitions of the preceding symbol.
(xyz)	Character group. Matches the characters xyz in that exact order.
\|	Alternation. Matches either the characters before or the characters after the symbol.
\	Escapes the next character. This allows you to match reserved characters `[ ] ( ) { } . * + ? ^ $ \ \|`
^	Matches the beginning of the input.
$	Matches the end of the input.

2.1 The Full Stop

The full stop . is the simplest example of a meta character. The meta character . matches any single character. It will not match return or newline characters. For example, the regular expression .ar means: any character, followed by the letter a, followed by the letter r.

".ar" => The car parked in the garage.

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2.2 Character Sets

Character sets are also called character classes. Square brackets are used to specify character sets. Use a hyphen inside a character set to specify the characters' range. The order of the character range inside the square brackets doesn't matter. For example, the regular expression [Tt]he means: an uppercase T or lowercase t, followed by the letter h, followed by the letter e.

"[Tt]he" => The car parked in the garage.

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A period inside a character set, however, means a literal period. The regular expression ar[.] means: a lowercase character a, followed by the letter r, followed by a period . character.

"ar[.]" => A garage is a good place to park a car.

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2.2.1 Negated Character Sets

In general, the caret symbol represents the start of the string, but when it is typed after the opening square bracket it negates the character set. For example, the regular expression [^c]ar means: any character except c, followed by the character a, followed by the letter r.

"[^c]ar" => The car parked in the garage.

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2.3 Repetitions

The meta characters +, * or ? are used to specify how many times a subpattern can occur. These meta characters act differently in different situations.

2.3.1 The Star

The * symbol matches zero or more repetitions of the preceding matcher. The regular expression a* means: zero or more repetitions of the preceding lowercase character a. But if it appears after a character set or class then it finds the repetitions of the whole character set. For example, the regular expression [a-z]* means: any number of lowercase letters in a row.

"[a-z]*" => The car parked in the garage #21.

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The * symbol can be used with the meta character . to match any string of characters .*. The * symbol can be used with the whitespace character \s to match a string of whitespace characters. For example, the expression \s*cat\s* means: zero or more spaces, followed by a lowercase c, followed by a lowercase a, followed by a lowercase t, followed by zero or more spaces.

"\s*cat\s*" => The fat cat sat on the concatenation.

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2.3.2 The Plus

The + symbol matches one or more repetitions of the preceding character. For example, the regular expression c.+t means: a lowercase c, followed by at least one character, followed by a lowercase t. It needs to be clarified thatt is the last t in the sentence.

"c.+t" => The fat cat sat on the mat.

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2.3.3 The Question Mark

In regular expressions, the meta character ? makes the preceding character optional. This symbol matches zero or one instance of the preceding character. For example, the regular expression [T]?he means: Optional uppercase T, followed by a lowercase h, followed bya lowercase e.

"[T]he" => The car is parked in the garage.

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"[T]?he" => The car is parked in the garage.

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2.4 Braces

In regular expressions, braces (also called quantifiers) are used to specify the number of times that a character or a group of characters can be repeated. For example, the regular expression [0-9]{2,3} means: Match at least 2 digits, but not more than 3, ranging from 0 to 9.

"[0-9]{2,3}" => The number was 9.9997 but we rounded it off to 10.0.

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We can leave out the second number. For example, the regular expression [0-9]{2,} means: Match 2 or more digits. If we also remove the comma, the regular expression [0-9]{3} means: Match exactly 3 digits.

"[0-9]{2,}" => The number was 9.9997 but we rounded it off to 10.0.

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"[0-9]{3}" => The number was 9.9997 but we rounded it off to 10.0.

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2.5 Capturing Groups

A capturing group is a group of subpatterns that is written inside parentheses (...). As discussed before, in regular expressions, if we put a quantifier after a character then it will repeat the preceding character. But if we put a quantifier after a capturing group then it repeats the whole capturing group. For example, the regular expression (ab)* matches zero or more repetitions of the character "ab". We can also use the alternation | meta character inside a capturing group. For example, the regular expression (c|g|p)ar means: a lowercase c, g or p, followed by a, followed by r.

"(c|g|p)ar" => The car is parked in the garage.

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Note that capturing groups do not only match, but also capture, the characters for use in the parent language. The parent language could be Python or JavaScript or virtually any language that implements regular expressions in a function definition.

2.5.1 Non-Capturing Groups

A non-capturing group is a capturing group that matches the characters but does not capture the group. A non-capturing group is denoted by a ? followed by a : within parentheses (...). For example, the regular expression (?:c|g|p)ar is similar to (c|g|p)ar in that it matches the same characters but will not create a capture group.

"(?:c|g|p)ar" => The car is parked in the garage.

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Non-capturing groups can come in handy when used in find-and-replace functionality or when mixed with capturing groups to keep the overview when producing any other kind of output. See also 4. Lookaround.

2.6 Alternation

In a regular expression, the vertical bar | is used to define alternation. Alternation is like an OR statement between multiple expressions. Now, you may be thinking that character sets and alternation work the same way. But the big difference between character sets and alternation is that character sets work at the character level but alternation works at the expression level. For example, the regular expression (T|t)he|car means: either (an uppercase T or a lowercase t, followed by a lowercase h, followed by a lowercase e) OR (a lowercase c, followed by a lowercase a, followed by a lowercase r). Note that I included the parentheses for clarity, to show that either expression in parentheses can be met and it will match.

"(T|t)he|car" => The car is parked in the garage.

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2.7 Escaping Special Characters

A backslash \ is used in regular expressions to escape the next character. This allows us to include reserved characters such as { } [ ] / \ + * . $ ^ | ? as matching characters. To use one of these special character as a matching character, prepend it with \.

For example, the regular expression . is used to match any character except a newline. Now, to match . in an input string, the regular expression (f|c|m)at\.? means: a lowercase f, c or m, followed by a lowercase a, followed by a lowercase t, followed by an optional . character.

"(f|c|m)at\.?" => The fat cat sat on the mat.

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2.8 Anchors

In regular expressions, we use anchors to check if the matching symbol is the starting symbol or ending symbol of the input string. Anchors are of two types: The first type is the caret ^ that checks if the matching character is the first character of the input and the second type is the dollar sign $ which checks if a matching character is the last character of the input string.

2.8.1 The Caret

The caret symbol ^ is used to check if a matching character is the first character of the input string. If we apply the following regular expression ^a (meaning 'a' must be the starting character) to the string abc, it will match a. But if we apply the regular expression ^b to the above string, it will not match anything. Because in the string abc, the "b" is not the starting character. Let's take a look at another regular expression ^(T|t)he which means: an uppercase T or a lowercase t must be the first character in the string, followed by a lowercase h, followed by a lowercase e.

"(T|t)he" => The car is parked in the garage.

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"^(T|t)he" => The car is parked in the garage.

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2.8.2 The Dollar Sign

The dollar sign $ is used to check if a matching character is the last character in the string. For example, the regular expression (at\.)$ means: a lowercase a, followed by a lowercase t, followed by a . character and the matcher must be at the end of the string.

"(at\.)" => The fat cat. sat. on the mat.

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"(at\.)$" => The fat cat. sat. on the mat.

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3. Shorthand Character Sets

There are a number of convenient shorthands for commonly used character sets/ regular expressions:

Shorthand	Description
.	Any character except new line
\w	Matches alphanumeric characters: `[a-zA-Z0-9_]`
\W	Matches non-alphanumeric characters: `[^\w]`
\d	Matches digits: `[0-9]`
\D	Matches non-digits: `[^\d]`
\s	Matches whitespace characters: `[\t\n\f\r\p{Z}]`
\S	Matches non-whitespace characters: `[^\s]`

4. Lookarounds

Lookbehinds and lookaheads (also called lookarounds) are specific types of non-capturing groups (used to match a pattern but without including it in the matching list). Lookarounds are used when we a pattern must be preceded or followed by another pattern. For example, imagine we want to get all numbers that are preceded by the $ character from the string $4.44 and $10.88. We will use the following regular expression (?<=\$)[0-9\.]* which means: get all the numbers which contain the . character and are preceded by the $ character. These are the lookarounds that are used in regular expressions:

Symbol	Description
?=	Positive Lookahead
?!	Negative Lookahead
?<=	Positive Lookbehind
?<!	Negative Lookbehind

4.1 Positive Lookahead

The positive lookahead asserts that the first part of the expression must be followed by the lookahead expression. The returned match only contains the text that is matched by the first part of the expression. To define a positive lookahead, parentheses are used. Within those parentheses, a question mark with an equals sign is used like this: (?=...). The lookahead expressions is written after the equals sign inside parentheses. For example, the regular expression (T|t)he(?=\sfat) means: match either a lowercase t or an uppercase T, followed by the letter h, followed by the letter e. In parentheses we define a positive lookahead which tells the regular expression engine to match The or the only if it's followed by the word fat.

"(T|t)he(?=\sfat)" => The fat cat sat on the mat.

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4.2 Negative Lookahead

Negative lookaheads are used when we need to get all matches from an input string that are not followed by a certain pattern. A negative lookahead is written the same way as a positive lookahead. The only difference is, instead of an equals sign =, we use an exclamation mark ! to indicate negation i.e. (?!...). Let's take a look at the following regular expression (T|t)he(?!\sfat) which means: get all The or the words from the input string that are not followed by a space character and the word fat.

"(T|t)he(?!\sfat)" => The fat cat sat on the mat.

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4.3 Positive Lookbehind

Positive lookbehinds are used to get all the matches that are preceded by a specific pattern. Positive lookbehinds are written (?<=...). For example, the regular expression (?<=(T|t)he\s)(fat|mat) means: get all fat or mat words from the input string that come after the word The or the.

"(?<=(T|t)he\s)(fat|mat)" => The fat cat sat on the mat.

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4.4 Negative Lookbehind

Negative lookbehinds are used to get all the matches that are not preceded by a specific pattern. Negative lookbehinds are written (?<!...). For example, the regular expression (?<!(T|t)he\s)(cat) means: get all cat words from the input string that are not after the word The or the.

"(?<!(T|t)he\s)(cat)" => The cat sat on cat.

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5. Flags

Flags are also called modifiers because they modify the output of a regular expression. These flags can be used in any order or combination, and are an integral part of the RegExp.

Flag	Description
i	Case insensitive: Match will be case-insensitive.
g	Global Search: Match all instances, not just the first.
m	Multiline: Anchor meta characters work on each line.

5.1 Case Insensitive

The i modifier is used to perform case-insensitive matching. For example, the regular expression /The/gi means: an uppercase T, followed by a lowercase h, followed by an e. And at the end of regular expression the i flag tells the regular expression engine to ignore the case. As you can see, we also provided g flag because we want to search for the pattern in the whole input string.

"The" => The fat cat sat on the mat.

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"/The/gi" => The fat cat sat on the mat.

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5.2 Global Search

The g modifier is used to perform a global match (finds all matches rather than stopping after the first match). For example, the regular expression/.(at)/g means: any character except a new line, followed by a lowercase a, followed by a lowercase t. Because we provided the g flag at the end of the regular expression, it will now find all matches in the input string, not just the first one (which is the default behavior).

"/.(at)/" => The fat cat sat on the mat.

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"/.(at)/g" => The fat cat sat on the mat.

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5.3 Multiline

The m modifier is used to perform a multi-line match. As we discussed earlier, anchors (^, $) are used to check if a pattern is at the beginning of the input or the end. But if we want the anchors to work on each line, we use the m flag. For example, the regular expression /at(.)?$/gm means: a lowercase a, followed by a lowercase t and, optionally, anything except a new line. And because of the m flag, the regular expression engine now matches patterns at the end of each line in a string.

"/.at(.)?$/" => The fat
                cat sat
                on the mat.

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"/.at(.)?$/gm" => The fat
                  cat sat
                  on the mat.

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6. Greedy vs Lazy Matching

By default, a regex will perform a greedy match, which means the match will be as long as possible. We can use ? to match in a lazy way, which means the match should be as short as possible.

"/(.*at)/" => The fat cat sat on the mat.

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"/(.*?at)/" => The fat cat sat on the mat.

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MIT © Zeeshan Ahmad

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