+ +
+
+ +
+
+
+
+
+
+
+ 
+
+"the" => The fat cat sat on the mat. ++ +
+"The" => The fat cat sat on the mat. ++ +
+ 
+ [ ] ( ) { } . * + ? ^ $ \ ||
+|^|Matches the beginning of the input.|
+|$|Matches the end of the input.|
+
+## 2.1 Full stop
+
+Full stop `.` is the simplest example of 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. ++ +[Test the regular expression](https://regex101.com/r/xc9GkU/1) + +## 2.2 Character set + +Character sets are also called character class. 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 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. ++ +[Test the regular expression](https://regex101.com/r/2ITLQ4/1) + +A period inside a character set, however, means a literal period. The regular +expression `ar[.]` means: a lowercase character `a`, followed by letter `r`, +followed by a period `.` character. + +
+"ar[.]" => A garage is a good place to park a car. ++ +[Test the regular expression](https://regex101.com/r/wL3xtE/1) + +### 2.2.1 Negated character set + +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. ++ +[Test the regular expression](https://regex101.com/r/nNNlq3/1) + +## 2.3 Repetitions + +Following 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 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. ++ +[Test the regular expression](https://regex101.com/r/7m8me5/1) + +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 lowercase character `c`, +followed by lowercase character `a`, followed by lowercase character `t`, +followed by zero or more spaces. + +
+"\s*cat\s*" => The fat cat sat on the concatenation. ++ +[Test the regular expression](https://regex101.com/r/gGrwuz/1) + +### 2.3.2 The Plus + +The symbol `+` matches one or more repetitions of the preceding character. For +example, the regular expression `c.+t` means: lowercase letter `c`, followed by +at least one character, followed by the lowercase character `t`. It needs to be +clarified that `t` is the last `t` in the sentence. + +
+"c.+t" => The fat cat sat on the mat. ++ +[Test the regular expression](https://regex101.com/r/Dzf9Aa/1) + +### 2.3.3 The Question Mark + +In regular expression 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 the uppercase +letter `T`, followed by the lowercase character `h`, followed by the lowercase +character `e`. + +
+"[T]he" => The car is parked in the garage. ++ +[Test the regular expression](https://regex101.com/r/cIg9zm/1) + +
+"[T]?he" => The car is parked in the garage. ++ +[Test the regular expression](https://regex101.com/r/kPpO2x/1) + +## 2.4 Braces + +In regular expression braces that are 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 ( characters in the range of 0 to 9). + +
+"[0-9]{2,3}" => The number was 9.9997 but we rounded it off to 10.0.
+
+
+[Test the regular expression](https://regex101.com/r/juM86s/1)
+
+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.
+
+
+[Test the regular expression](https://regex101.com/r/Gdy4w5/1)
+
+
+"[0-9]{3}" => The number was 9.9997 but we rounded it off to 10.0.
+
+
+[Test the regular expression](https://regex101.com/r/Sivu30/1)
+
+## 2.5 Capturing Group
+
+A capturing group is a group of sub-patterns that is written inside Parentheses
+`(...)`. Like as we discussed before that in regular expression if we put a quantifier
+after a character then it will repeat the preceding character. But if we put 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 capturing group.
+For example, the regular expression `(c|g|p)ar` means: lowercase character `c`,
+`g` or `p`, followed by character `a`, followed by character `r`.
+
++"(c|g|p)ar" => The car is parked in the garage. ++ +[Test the regular expression](https://regex101.com/r/tUxrBG/1) + +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 group + +A non-capturing group is a capturing group that only matches the characters, but +does not capture the group. A non-capturing group is denoted by a `?` followed by a `:` +within parenthesis `(...)`. 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. ++ +[Test the regular expression](https://regex101.com/r/Rm7Me8/1) + +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](#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 set and alternation works the same way. But the big +difference between character set and alternation is that character set works on +character level but alternation works on expression level. For example, the +regular expression `(T|t)he|car` means: either (uppercase character `T` or lowercase +`t`, followed by lowercase character `h`, followed by lowercase character `e`) OR +(lowercase character `c`, followed by lowercase character `a`, followed by +lowercase character `r`). Note that I put 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. ++ +[Test the regular expression](https://regex101.com/r/fBXyX0/1) + +## 2.7 Escaping special character + +Backslash `\` is used in regular expression to escape the next character. This +allows us to specify a symbol as a matching character including reserved +characters `{ } [ ] / \ + * . $ ^ | ?`. To use a special character as a matching +character prepend `\` before it. + +For example, the regular expression `.` is used to match any character except +newline. Now to match `.` in an input string the regular expression +`(f|c|m)at\.?` means: lowercase letter `f`, `c` or `m`, followed by lowercase +character `a`, followed by lowercase letter `t`, followed by optional `.` +character. + +
+"(f|c|m)at\.?" => The fat cat sat on the mat. ++ +[Test the regular expression](https://regex101.com/r/DOc5Nu/1) + +## 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: +First type is Caret `^` that check if the matching character is the start +character of the input and the second type is Dollar `$` that checks if matching +character is the last character of the input string. + +### 2.8.1 Caret + +Caret `^` symbol is used to check if matching character is the first character +of the input string. If we apply the following regular expression `^a` (if a is +the starting symbol) to input string `abc` it matches `a`. But if we apply +regular expression `^b` on above input string it does not match anything. +Because in input string `abc` "b" is not the starting symbol. Let's take a look +at another regular expression `^(T|t)he` which means: uppercase character `T` or +lowercase character `t` is the start symbol of the input string, followed by +lowercase character `h`, followed by lowercase character `e`. + +
+"(T|t)he" => The car is parked in the garage. ++ +[Test the regular expression](https://regex101.com/r/5ljjgB/1) + +
+"^(T|t)he" => The car is parked in the garage. ++ +[Test the regular expression](https://regex101.com/r/jXrKne/1) + +### 2.8.2 Dollar + +Dollar `$` symbol is used to check if matching character is the last character +of the input string. For example, regular expression `(at\.)$` means: a +lowercase character `a`, followed by lowercase character `t`, followed by a `.` +character and the matcher must be end of the string. + +
+"(at\.)" => The fat cat. sat. on the mat. ++ +[Test the regular expression](https://regex101.com/r/y4Au4D/1) + +
+"(at\.)$" => The fat cat. sat. on the mat. ++ +[Test the regular expression](https://regex101.com/r/t0AkOd/1) + +## 3. Shorthand Character Sets + +Regular expression provides shorthands for the commonly used character sets, +which offer convenient shorthands for commonly used regular expressions. The +shorthand character sets are as follows: + +|Shorthand|Description| +|:----:|----| +|.|Any character except new line| +|\w|Matches alphanumeric characters: `[a-zA-Z0-9_]`| +|\W|Matches non-alphanumeric characters: `[^\w]`| +|\d|Matches digit: `[0-9]`| +|\D|Matches non-digit: `[^\d]`| +|\s|Matches whitespace character: `[\t\n\f\r\p{Z}]`| +|\S|Matches non-whitespace character: `[^\s]`| + +## 4. Lookaround + +Lookbehind and lookahead (also called lookaround) are specific types of +***non-capturing groups*** (Used to match the pattern but not included in matching +list). Lookarounds are used when we have the condition that this pattern is +preceded or followed by another certain pattern. For example, we want to get all +numbers that are preceded by `$` character from the following input string +`$4.44 and $10.88`. We will use following regular expression `(?<=\$)[0-9\.]*` +which means: get all the numbers which contain `.` character and are preceded +by `$` character. Following are the lookarounds that are used in regular +expressions: + +|Symbol|Description| +|:----:|----| +|?=|Positive Lookahead| +|?!|Negative Lookahead| +|?<=|Positive Lookbehind| +|? +"(T|t)he(?=\sfat)" => The fat cat sat on the mat. + + +[Test the regular expression](https://regex101.com/r/IDDARt/1) + +### 4.2 Negative Lookahead + +Negative lookahead is used when we need to get all matches from input string +that are not followed by a pattern. Negative lookahead is defined same as we define +positive lookahead but the only difference is instead of equal `=` character we +use negation `!` character 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 input string that are not followed by the word `fat` precedes by a space +character. + +
+"(T|t)he(?!\sfat)" => The fat cat sat on the mat. ++ +[Test the regular expression](https://regex101.com/r/V32Npg/1) + +### 4.3 Positive Lookbehind + +Positive lookbehind is used to get all the matches that are preceded by a +specific pattern. Positive lookbehind is denoted by `(?<=...)`. For example, the +regular expression `(?<=(T|t)he\s)(fat|mat)` means: get all `fat` or `mat` words +from input string that are after the word `The` or `the`. + +
+"(?<=(T|t)he\s)(fat|mat)" => The fat cat sat on the mat. ++ +[Test the regular expression](https://regex101.com/r/avH165/1) + +### 4.4 Negative Lookbehind + +Negative lookbehind is used to get all the matches that are not preceded by a +specific pattern. Negative lookbehind is denoted by `(? +"(?<!(T|t)he\s)(cat)" => The cat sat on cat. + + +[Test the regular expression](https://regex101.com/r/8Efx5G/1) + +## 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: Sets matching to be case-insensitive.| +|g|Global Search: Search for a pattern throughout the input string.| +|m|Multiline: Anchor meta character works 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: uppercase letter `T`, followed by lowercase +character `h`, followed by character `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. ++ +[Test the regular expression](https://regex101.com/r/dpQyf9/1) + +
+"/The/gi" => The fat cat sat on the mat. ++ +[Test the regular expression](https://regex101.com/r/ahfiuh/1) + +### 5.2 Global search + +The `g` modifier is used to perform a global match (find all matches rather than +stopping after the first match). For example, the regular expression`/.(at)/g` +means: any character except new line, followed by lowercase character `a`, +followed by lowercase character `t`. Because we provided `g` flag at the end of +the regular expression now it will 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. ++ +[Test the regular expression](https://regex101.com/r/jnk6gM/1) + +
+"/.(at)/g" => The fat cat sat on the mat. ++ +[Test the regular expression](https://regex101.com/r/dO1nef/1) + +### 5.3 Multiline + +The `m` modifier is used to perform a multi-line match. As we discussed earlier +anchors `(^, $)` are used to check if pattern is the beginning of the input or +end of the input string. But if we want that anchors works on each line we use +`m` flag. For example, the regular expression `/at(.)?$/gm` means: lowercase +character `a`, followed by lowercase character `t`, optionally anything except +new line. And because of `m` flag now regular expression engine matches pattern +at the end of each line in a string. + +
+"/.at(.)?$/" => The fat + cat sat + on the mat. ++ +[Test the regular expression](https://regex101.com/r/hoGMkP/1) + +
+"/.at(.)?$/gm" => The fat + cat sat + on the mat. ++ +[Test the regular expression](https://regex101.com/r/E88WE2/1) + +## 6. Greedy vs lazy matching +By default regex will do greedy matching , means it will match as long as +possible. we can use `?` to match in lazy way means as short as possible + +
+"/(.*at)/" => The fat cat sat on the mat.+ + +[Test the regular expression](https://regex101.com/r/AyAdgJ/1) + +
+"/(.*?at)/" => The fat cat sat on the mat.+ + +[Test the regular expression](https://regex101.com/r/AyAdgJ/2) + + +## Contribution + +* Open pull request with improvements +* Discuss ideas in issues +* Spread the word +* Reach out with any feedback [](https://twitter.com/ziishaned) + +## License + +MIT © [Zeeshan Ahmad](https://twitter.com/ziishaned) diff --git a/translations/README-fr.md b/translations/README-fr.md index 4d495e9..c51668c 100644 --- a/translations/README-fr.md +++ b/translations/README-fr.md @@ -30,6 +30,7 @@ * [Polish](../translations/README-pl.md) * [Русский](../translations/README-ru.md) * [Tiếng Việt](../translations/README-vn.md) +* [فارسی](../translations/README-fa.md) ## Qu'est-ce qu'une expression régulière? diff --git a/translations/README-gr.md b/translations/README-gr.md index 7d62a6e..2e6dd9d 100644 --- a/translations/README-gr.md +++ b/translations/README-gr.md @@ -30,6 +30,7 @@ * [Polish](../translations/README-pl.md) * [Русский](../translations/README-ru.md) * [Tiếng Việt](../translations/README-vn.md) +* [فارسی](../translations/README-fa.md) ## Τι είναι μια Κανονική Έκφραση (Regular Expression); diff --git a/translations/README-hu.md b/translations/README-hu.md index 1017260..e8c5239 100644 --- a/translations/README-hu.md +++ b/translations/README-hu.md @@ -30,6 +30,7 @@ * [Polish](../translations/README-pl.md) * [Русский](../translations/README-ru.md) * [Tiếng Việt](../translations/README-vn.md) +* [فارسی](../translations/README-fa.md) ## Mi az a reguláris kifejezés? diff --git a/translations/README-ja.md b/translations/README-ja.md index cb8df84..6acf9ae 100644 --- a/translations/README-ja.md +++ b/translations/README-ja.md @@ -30,6 +30,7 @@ * [Polish](../translations/README-pl.md) * [Русский](../translations/README-ru.md) * [Tiếng Việt](../translations/README-vn.md) +* [فارسی](../translations/README-fa.md) ## 正規表現とは diff --git a/translations/README-ko.md b/translations/README-ko.md index 05ae297..d4c9c0b 100644 --- a/translations/README-ko.md +++ b/translations/README-ko.md @@ -30,6 +30,7 @@ * [Polish](../translations/README-pl.md) * [Русский](../translations/README-ru.md) * [Tiếng Việt](../translations/README-vn.md) +* [فارسی](../translations/README-fa.md) ## 정규표현식이란 무엇인가? diff --git a/translations/README-pl.md b/translations/README-pl.md index c5213f5..fd1d04c 100644 --- a/translations/README-pl.md +++ b/translations/README-pl.md @@ -30,6 +30,7 @@ * [Polish](../translations/README-pl.md) * [Русский](../translations/README-ru.md) * [Tiếng Việt](../translations/README-vn.md) +* [فارسی](../translations/README-fa.md) ## Co to jest wyrażenie regularne? diff --git a/translations/README-pt_BR.md b/translations/README-pt_BR.md index 90170f4..774cd21 100644 --- a/translations/README-pt_BR.md +++ b/translations/README-pt_BR.md @@ -30,6 +30,7 @@ * [Polish](../translations/README-pl.md) * [Русский](../translations/README-ru.md) * [Tiếng Việt](../translations/README-vn.md) +* [فارسی](../translations/README-fa.md) ## O que é uma Expressão Regular? diff --git a/translations/README-ru.md b/translations/README-ru.md index 8e7ccde..ed2e668 100644 --- a/translations/README-ru.md +++ b/translations/README-ru.md @@ -29,6 +29,7 @@ * [Polish](../translations/README-pl.md) * [Русский](../translations/README-ru.md) * [Tiếng Việt](../translations/README-vn.md) +* [فارسی](../translations/README-fa.md) ## Что такое Регулярное выражение? diff --git a/translations/README-tr.md b/translations/README-tr.md index 8e60118..41be9dd 100644 --- a/translations/README-tr.md +++ b/translations/README-tr.md @@ -30,6 +30,7 @@ * [Polish](../translations/README-pl.md) * [Русский](../translations/README-ru.md) * [Tiếng Việt](../translations/README-vn.md) +* [فارسی](../translations/README-fa.md) ## Düzenli İfade Nedir? diff --git a/translations/README-vn.md b/translations/README-vn.md index 60c51e8..0c38faa 100644 --- a/translations/README-vn.md +++ b/translations/README-vn.md @@ -31,6 +31,7 @@ * [Polish](../translations/README-pl.md) * [Русский](../translations/README-ru.md) * [Tiếng Việt](../translations/README-vn.md) +* [فارسی](../translations/README-fa.md) ## Biểu thức chính quy là gì? diff --git a/translations/README-zh-simple.md b/translations/README-zh-simple.md index 8ee4847..292e877 100644 --- a/translations/README-zh-simple.md +++ b/translations/README-zh-simple.md @@ -29,6 +29,7 @@ * [Polish](../translations/README-pl.md) * [Русский](../translations/README-ru.md) * [Tiếng Việt](../translations/README-vn.md) +* [فارسی](../translations/README-fa.md) ## 什么是正则表达式?