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<br/>
<p align="center">
<img src="https://i.imgur.com/bYwl7Vf.png" alt="Learn Regex">
</p><br/>
## Translations:
* [English](README.md)
* [中文版](README-cn.md)
* [Español](README-es.md)
## What is Regular Expression?
> Una expresión regular es un grupo de caracteres o símbolos, los cuales son usados para buscar un patrón específico dentro de un texto.
Una expresión regular es un patrón que que se compara con una cadena de caracteres de izquierda a derecha. La palabra "expresión regular", puede también ser escrita como "Regex" o "Regexp". Las expresiones regulares se utiliza para remplazar un texto, dentro de un *string* (o cadena de caracteres), validar el formato, extraer un substring de un string completo basado en la coincidencia de una patrón, y muchas cosas más.
Imagina que estas escribiendo una aplicación y quieres agregar reglas para cuando el usuario elija su nombre de usuario. Nosotros vamos a querer que el nombre de usuario contenga letras, números, guión bajo, y guíon medio. También vamos a querer limitar el número de caracteres en el nombre de usuario para que no se vea feo. Para ello usamos la siguiente expresión regular para validar el nombre de usuario
<br/><br/>
<p align="center">
<img src="http://imgur.com/EtlKH14.png" alt="Regular expression">
</p>
De la expresión regular anterior, se puede aceptar las cadenas 'john_doe', 'jo-hn_doe' y 'john12_as'. La expresión no coincide con el nombre de usuario 'Jo', porque es una cadena de caracteres que contiene letras mayúsculas y es demasiado corta.
## Tabla de contenido
- [Introducción](#1-introduccion)
- [Meta caracteres](#2-meta-caracteres)
- [Full stop](#21-full-stop)
- [Conjunto de caracteres](#22-conjunto-de-caracteres)
- [Conjunto de caracteres negados](#221-conjunto-de-caracteres-negado)
- [Repeticiones](#23-repeticiones)
- [Asterísco](#231-asterisco)
- [Signo más](#232-signo-mas)
- [Signo de pregunta](#233-signo-de-pregunta)
- [Llaves](#24-llaves)
- [Grupo de caracteres](#25-grupo-de-caracteres)
- [Alternancia](#26-alternacia)
- [Caracteres especiales de escape](#27-caracteres-especiales-de-escape)
- [Anclas](#28-anclas)
- [Símbolo de intercalación](#281-simbolo-de-intercalacion)
- [Símbolo dolar](#282-simbolo-dolar)
- [Conjunto de caracteres abreviados](#3-conjunto-de-caracteres-abreviados)
- [Mirar alrededor](#4-mirar-alrededor)
- [Mirar hacia delante positivo](#41-mirar-hacia-delante-positivo)
- [Mirar hacia delante negativo](#41-mirar-hacia-delaten-negativo)
- [Mirar hacia atrás positivo](#41-mirar-hacia-atras-positivo)
- [Mirar hacia atrás negativo](#41-mirar-hacia-atras-negativo)
- [Banderas](#5-banderas)
- [mayúsculas y minúsculas](#51-mayusculas-y-minusculas)
- [Búsqueda global](#52-busqueda-global)
- [Multilinea](#53-multilinea)
- [Bonus](#bonus)
## 1. Introducción
Una expresión regular es sólo un patrón de caracteres que utilizamos para realizar búsquedas en un texto. Por ejemplo, la expresión regular «the» significa: la letra `t` seguida de la letra `h` seguida de la letra `e`.
<pre>
"the" => The fat cat sat on <a href="#learn-regex"><strong>the</strong></a> mat.
</pre>
[Prueba la expresión regular](https://regex101.com/r/dmRygT/1)
La expresión regular `123` coincide con la cadena `123`. La expresión regular se compara con una cadena de entrada al comparar cada carácter de la expresión regular con cada carácter de la cadena de entrada, uno tras otro. Las expresiones regulares son normalmente sensibles a mayúsculas y minúsculas, por lo que la expresión regular `The` no coincide con la cadena `the`.
<pre>
"The" => <a href="#learn-regex"><strong>The</strong></a> fat cat sat on the mat.
</pre>
[Prueba la expresión regular](https://regex101.com/r/1paXsy/1)
## 2. Meta caracteres
Los caracteres meta son los bloques de construcción de las expresiones regulares. Los meta caracteres no se sostienen a sí mismos, sino que se interpretan de alguna manera especial. Algunos meta caracteres tienen un significado especial y se escriben entre corchetes. Los meta caracteres son los siguientes:
|Meta character|Description|
|:----:|----|
|.|Periodo. Coincide con cualquier caracter excepto un salto de línea.|
|[ ]|Clase caracter. Coincide con cualquier caracter contenido entre corchetes.|
|[^ ]|Clase caracter negado. Coincide con cualquier caracter que no está contenido dentro de los corchetes.|
|*|Corresponde con 0 o más repeticiones del símbolo precedente.|
|+|Corresponde con 1 o más repeticiones del símbolo precedente.|
|?|Hace que el símbolo precedente sea opcional.|
|{n,m}|Llaves.Corresponde al menos "n" pero no más de "m" repeticiones del símbolo precedente.|
|(xyz)|Grupo caracter. Hace coincidir los caracteres xyz en ese orden exacto.|
|&#124;|Alternancia. Corresponde a los caracteres anteriores o los caracteres después del símbolo.|
|&#92;|Escapa el siguiente caracter. Esto le permite hacer coincidir los caracteres reservados <code>[ ] ( ) { } . * + ? ^ $ \ &#124;</code>|
|^|Hace coincidir el principio de la entrada.|
|$|Corresponde al final de la entrada.|
## 2.1 Full stop
Full stop `.` es el ejemplo más simple del meta-caracter. El caracter meta "." coincide con cualquier carácter. No coincidirá con el retorno o nuevos caracteres de línea. Por ejemplo, la expresión regular `.ar` significa: cualquier caracter, seguido de la letra`a`, seguido de la letra "r".
<pre>
".ar" => The <a href="#learn-regex"><strong>car</strong></a> <a href="#learn-regex"><strong>par</strong></a>ked in the <a href="#learn-regex"><strong>gar</strong></a>age.
</pre>
[Prueba la expresión regular](https://regex101.com/r/xc9GkU/1)
## 2.2 Conjunto de caracteres
Los conjuntos de caracteres también se llaman clase de caracteres. Los corchetes se utilizan para especificar conjuntos de caracteres. Utilice un guión dentro de un conjunto de caracteres para especificar el rango de los caracteres. El orden del rango de caracteres dentro de corchetes no importa. Por ejemplo, la expresión regular "[Tt] he" significa: una letra mayúscula "T" o <minúscula> t, seguida de la letra "h" seguida de la letra "e"
<pre>
"[Tt]he" => <a href="#learn-regex"><strong>The</strong></a> car parked in <a href="#learn-regex"><strong>the</strong></a> garage.
</pre>
[Prueba la expresión regular](https://regex101.com/r/2ITLQ4/1)
Sin embargo, un período dentro de un conjunto de caracteres significa un período literal. La expresión regular `ar [.]` Significa: un carácter minúsculo `a`, seguido de la letra` r`, seguido de un carácter `.`.
<pre>
"ar[.]" => A garage is a good place to park a c<a href="#learn-regex"><strong>ar.</strong></a>
</pre>
[Prueba la expresión regular](https://regex101.com/r/wL3xtE/1)
### 2.2.1 Conjunto de caracteres negados
En general, el símbolo de intercalación representa el comienzo de la cadena, pero cuando se escribe después del corchete de apertura niega el conjunto de caracteres. Por ejemplo, la expresión regular `[^c] ar` significa: cualquier carácter, excepto `c`, seguido del carácter `a`, seguido de la letra `r`.
<pre>
"[^c]ar" => The car <a href="#learn-regex"><strong>par</strong></a>ked in the <a href="#learn-regex"><strong>gar</strong></a>age.
</pre>
[Prueba la expresión regular](https://regex101.com/r/nNNlq3/1)
## 2.3 Repeticiones
Siguiendo los caracteres meta +, * o ?, se utilizan para especificar cuántas veces puede producirse un subpatrón. Estos meta-caracteres actúan de manera diferente en diferentes situaciones.
### 2.3.1 Asterísco
El símbolo `*` coincide con cero o más repeticiones del marcador anterior. La expresión regular `a*` significa: cero o más repeticiones del carácter en minúscula precedente `a`. Pero si aparece después de un conjunto de caracteres o una clase, entonces encuentra las repeticiones de todo el conjunto de caracteres. Por ejemplo, la expresión regular `[a-z]*` significa: cualquier número de letras minúsculas en una fila.
<pre>
"[a-z]*" => T<a href="#learn-regex"><strong>he</strong></a> <a href="#learn-regex"><strong>car</strong></a> <a href="#learn-regex"><strong>parked</strong></a> <a href="#learn-regex"><strong>in</strong></a> <a href="#learn-regex"><strong>the</strong></a> <a href="#learn-regex"><strong>garage</strong></a> #21.
</pre>
[Prueba la expresión regular](https://regex101.com/r/7m8me5/1)
El símbolo `*` se puede utilizar con el meta-caracter `.` para que coincida con cualquier cadena de caracteres `.*`. El símbolo `*` se lo puede utilizar con el caracter de espacio en blanco `\s` para que coincida con una cadena de caracteres de espacio en blanco. Por ejemplo, la expresión "\s*cat\s*" significa: cero o más espacios, seguido por el carácter en minúscula `c`, seguido del carácter en minúscula `a`, seguido del carácter en minúscula `t`, seguido de cero o más espacios.
<pre>
"\s*cat\s*" => The fat<a href="#learn-regex"><strong> cat </strong></a>sat on the <a href="#learn-regex">con<strong>cat</strong>enation</a>.
</pre>
[Prueba la expresión regular](https://regex101.com/r/gGrwuz/1)
### 2.3.2 Signo más
El símbolo `+` coincide con una o más repeticiones del carácter anterior. Por ejemplo, la expresión regular `c.+T` significa: letra en minúscula `c`, seguida por al menos uno del mismo carácter, luego el carácter en minúscula `t`.
<pre>
"c.+t" => The fat <a href="#learn-regex"><strong>cat sat on the mat</strong></a>.
</pre>
[Prueba la expresión regular](https://regex101.com/r/Dzf9Aa/1)
### 2.3.3 Signo de pregunta
En expresiones regulares el meta-caracter `?` hace que el caracter precedente sea opcional. Este símnbolo coincide con cero o una instancia del caracter precedente. Por ejemplo, la expresión regular `[T]?he` significa: El caracteropcional predecesor `T` seguido por la letra en minúscula `h`, seguido del caracter en minúscula `e`.
<pre>
"[T]he" => <a href="#learn-regex"><strong>The</strong></a> car is parked in the garage.
</pre>
[Prueba la expresión regular](https://regex101.com/r/cIg9zm/1)
<pre>
"[T]?he" => <a href="#learn-regex"><strong>The</strong></a> car is parked in t<a href="#learn-regex"><strong>he</strong></a> garage.
</pre>
[Prueba la expresión regular](https://regex101.com/r/kPpO2x/1)
## 2.4 Llaves
En la expresión regular, las llaves que también se denominan cuantificadores se utilizan para especificar el número de veces que se puede repetir un carácter o un grupo de caracteres. Por ejemplo, la expresión regular `[0-9]{2,3}` significa: Combina al menos 2 dígitos pero no más de 3 (caracteres del rango de 0 a 9).
<pre>
"[0-9]{2,3}" => The number was 9.<a href="#learn-regex"><strong>999</strong></a>7 but we rounded it off to <a href="#learn-regex"><strong>10</strong></a>.0.
</pre>
[Prueba la expresión regular](https://regex101.com/r/juM86s/1)
Podemos dejar fuera el segundo número. Por ejemplo, la expresión regular `[0-9] {2,}` significa: Combina 2 o más dígitos. Si también eliminamos la coma, la expresión regular `[0-9]{3}` significa: coincidir exactamente con 3 dígitos.
<pre>
"[0-9]{2,}" => The number was 9.<a href="#learn-regex"><strong>9997</strong></a> but we rounded it off to <a href="#learn-regex"><strong>10</strong></a>.0.
</pre>
[Prueba la expresión regular](https://regex101.com/r/Gdy4w5/1)
<pre>
"[0-9]{3}" => The number was 9.<a href="#learn-regex"><strong>999</strong></a>7 but we rounded it off to 10.0.
</pre>
[Prueba la expresión regular](https://regex101.com/r/Sivu30/1)
## 2.5 Grupos de caracteres
Grupo de caracteres es un grupo de sub-patrones que se escribe dentro de paréntesis `(...)`. Como hemos discutido antes en la expresión regular si ponemos un cuantificador después de un caracter, repetiremos el caracter anterior. Pero si ponemos cuantificador después de un grupo de caracteres, entonces repetimos todo el grupo de caracteres. Por ejemplo, la expresión regular `(ab)*` coincide con cero o más repeticiones del caracter "ab". También podemos usar el caracter de alternancia `|` meta dentro del grupo de caracteres. Por ejemplo, la expresión regular `(c|g|p)ar` significa: caracter en minúscula `c`, `g` o `p`, seguido del caracter `a`, seguido del caracter `r`.
<pre>
"(c|g|p)ar" => The <a href="#learn-regex"><strong>car</strong></a> is <a href="#learn-regex"><strong>par</strong></a>ked in the <a href="#learn-regex"><strong>gar</strong></a>age.
</pre>
[Prueba la expresión regular](https://regex101.com/r/tUxrBG/1)
## 2.6 Alternancia
En la expresión regular se usa la barra vertical `|` para definir la alternancia. La alternancia es como una condición entre múltiples expresiones. Ahora, puedes estar pensando que el conjunto de caracteres y la alternancia funciona de la misma manera. Pero la gran diferencia entre el conjunto de caracteres y la alternancia es que el conjunto de caracteres funciona a nivel de caracter pero la alternancia funciona a nivel de expresión. Por ejemplo, la expresión regular `(T|t)he|car` significa: el carcter en mayúscula `T` o en minúscula `t`, seguido del caracter en minúscula `h`, seguido del caracter en minúscula `e` o del caracter en minúscula `c`, seguido de un caracter en minúscula `a`, seguido del carácter en minúscula `r`.
<pre>
"(T|t)he|car" => <a href="#learn-regex"><strong>The</strong></a> <a href="#learn-regex"><strong>car</strong></a> is parked in <a href="#learn-regex"><strong>the</strong></a> garage.
</pre>
[Prueba la expresión regular](https://regex101.com/r/fBXyX0/1)
## 2.7 Caracteres especiales de escape
La barra invertida `\` se utiliza en la expresión regular para escapar del carácter siguiente. Esto permite especificar un símbolo como un caracter coincidente incluyendo caracteres reservados `{}[]/\+*.^|?`. Por ejemplo, la expresión regular `.` se utiliza para coincidir con cualquier caracter, excepto la nueva línea. Ahora, para emparejar `.` en una cadena de entrada, la expresión regular `(f|c|m)at\.?` significa: la letra minúscula `f`, `c` o `m`, seguida del caracter en minúscula `a`, seguido de la letra minúscula `t`, seguida del caracter opcional `.`.
<pre>
"(f|c|m)at\.?" => The <a href="#learn-regex"><strong>fat</strong></a> <a href="#learn-regex"><strong>cat</strong></a> sat on the <a href="#learn-regex"><strong>mat.</strong></a>
</pre>
[Prueba la expresión regular](https://regex101.com/r/DOc5Nu/1)
## 2.8 Anclas
En expresiones regulares, usamos anclas para comprobar si el símbolo de coincidencia es el símbolo inicial o el símbolo final de la cadena de entrada. Los anclajes son de dos tipos: El primer tipo es el símbolo `^` que comprueba si el caracter coincidente es el caracter inicial de la entrada y el segundo tipo es Dollar `$` que comprueba si el caracter coincidente es el último caracter de la cadena de entrada.
### 2.8.1 Simbolo de intercalación
El símbolo de intercalación `^` se usa para verificar si el caracter coincidente es el primer caracter de la cadena de entrada. Si aplicamos la siguiente expresión regular `^a` (si a es el símbolo inicial) a la cadena de entrada `abc` coincide con `a`. Pero si aplicamos la expresión regular `^b` en la cadena de entrada anterior, no coincide con nada. Porque en la cadena de entrada `abc` "b" no es el símbolo inicial. Vamos a echar un vistazo a otra expresión regular `^(T|t)he`, significa: mayúsculas `T` o la letra minúscula `t` es el símbolo inicial de la cadena de entrada, seguido del caracter minúscula `h` y seguido del caracter en minúscula `e`.
<pre>
"(T|t)he" => <a href="#learn-regex"><strong>The</strong></a> car is parked in <a href="#learn-regex"><strong>the</strong></a> garage.
</pre>
[Prueba la expresión regular](https://regex101.com/r/5ljjgB/1)
<pre>
"^(T|t)he" => <a href="#learn-regex"><strong>The</strong></a> car is parked in the garage.
</pre>
[Prueba la expresión regular](https://regex101.com/r/jXrKne/1)
### 2.8.2 Símbolo dolar
El símbolo de dólar `$` se utiliza para comprobar si el caracter coincidente es el último carácter de la cadena de entrada. Por ejemplo, la expresión regular `(at\.)$` significa: un caracter en minúscula `a`, seguido del caracter en minúscula `t` seguido de un carácter `.` y el marcador debe ser el final de la cadena.
<pre>
"(at\.)" => The fat c<a href="#learn-regex"><strong>at.</strong></a> s<a href="#learn-regex"><strong>at.</strong></a> on the m<a href="#learn-regex"><strong>at.</strong></a>
</pre>
[Prueba la expresión regular](https://regex101.com/r/y4Au4D/1)
<pre>
"(at\.)$" => The fat cat. sat. on the m<a href="#learn-regex"><strong>at.</strong></a>
</pre>
[Pueba la expresión regular](https://regex101.com/r/t0AkOd/1)
## 3. Conjunto de caracteres abreviados
La expresión regular proporciona abreviaturas para los conjuntos de caracteres
comúnmente utilizados, que ofrecen abreviaturas convenientes para expresiones
regulares de uso común. Los conjuntos de caracteres abreviados son los siguientes:
|Shorthand|Description|
|:----:|----|
|.|Cualquier caracter excepto la nueva línea|
|\w|Coincide con los caracteres alfanuméricos: `[a-zA-Z0-9_]`|
|\W|Coincide con los caracteres no alfanuméricos: `[^\w]`|
|\d|Coincide con dígitos: `[0-9]`|
|\D|Coincide con no dígitos: `[^\d]`|
|\s|Coincide con caracteres espaciales: `[\t\n\f\r\p{Z}]`|
|\S|Coincide con caracteres no espaciales: `[^\s]`|
## 4. Mirar alrededor
Mirar hacia delante (lookaheds) y mirar hacia atrás (Lookbehind) a veces conocidos
como lookaround son tipo específico de ***grupo que no captura*** (Utilice para
coincidir con el patrón pero no se incluye en la lista correspondiente). Los
lookaheads se usan cuando tenemos la condición de que este patrón es precedido o
seguido por otro patrón determinado. Por ejemplo, queremos obtener todos los números
que están precedidos por el carácter `$` de la siguiente cadena de entrada
`$4.44 y $10.88`. Usaremos la siguiente expresión regular `(?<=\$)[0-9\.] *`,
esto significa: obtener todos los números que contienen el carácter `.` y
están precedidos del carácter `$`. A continuación se muestran los lookarounds
que se utilizan en expresiones regulares:
|Symbol|Description|
|:----:|----|
|?=|Positive Lookahead|
|?!|Negative Lookahead|
|?<=|Positive Lookbehind|
|?<!|Negative Lookbehind|
### 4.1 Mirar hacia adelate positiva
El lookahead positivo afirma que la primera parte de la expresión debe ser
seguida por la expresión lookahead. El matchonly devuelto contiene el texto que
coincide con la primera parte de la expresión. Para definir un lookahead positivo,
se utilizan paréntesis. Dentro de esos paréntesis, un signo de interrogación con
signo igual se utiliza de esta manera: `(?= ...)`. La expresión de Lookahead se
escribe después del signo igual dentro de los paréntesis. Por ejemplo, la
expresión regular `[T|t]he (?=\Sfat) significa: opcionalmente emparejar
la letra minúscula `t` o la letra mayúscula `T`, seguida de la letra `h`, seguida
de la letra `e`. Entre paréntesis definimos lookahead positivo que indica al motor
de expresión regular que coincida con `The` o` the` seguido de la palabra `fat`.
<pre>
"[T|t]he(?=\sfat)" => <a href="#learn-regex"><strong>The</strong></a> fat cat sat on the mat.
</pre>
[Prueba la expresión regular](https://regex101.com/r/IDDARt/1)
### 4.2 Mirar hacia adelate negativa
El lookahead negativo se usa cuando necesitamos obtener todas las coincidencias
de la cadena de entrada que no son seguidas por un patrón. El aspecto negativo se
define de la misma manera que definimos el aspecto positivo, pero la única diferencia
es que en lugar del caracter igual `=` utilizamos la negción `!` , es decir,
`(?! ...)`. Vamos a echar un vistazo a la siguiente expresión regular `[T|t]he(?!\Sfat)`
que significa: obtener todas las `The` o `the` seguidos por la palabra `fat` precedido por un carácter de espacio.
<pre>
"[T|t]he(?!\sfat)" => The fat cat sat on <a href="#learn-regex"><strong>the</strong></a> mat.
</pre>
[Prueba la expresión](https://regex101.com/r/V32Npg/1)
### 4.3 Mirar hacia atras positiva
Positivo lookbehind se utiliza para obtener todos los caracteres que están precedidos
por un patrón específico. La apariencia positiva se denomina `(?<=...)`.
Por ejemplo, la expresión regular `(? <= [T|t]he\s)(fat|mat)` significa: obtener todas las palabras
`fat` o `mat` de la cadena de entrada después de la palabra `The` o `the`.
<pre>
"(?<=[T|t]he\s)(fat|mat)" => The <a href="#learn-regex"><strong>fat</strong></a> cat sat on the <a href="#learn-regex"><strong>mat</strong></a>.
</pre>
[Prueba la expresión regular](https://regex101.com/r/avH165/1)
### 4.4 Mirar hacia atras negativa
El lookbehind negativo se utiliza para obtener todas las coincidencias que no
están precedidas por un patrón específico. El lookbehind negativo se denota por
`(? <! ...)`. Por ejemplo, la expresión regular `(?<!(T|t)he(s)(cat)` significa:
obtener todas las palabras `cat` de la cadena de entrada que no están después de
la palabra` The` o `the`.
<pre>
"(?&lt;![T|t]he\s)(cat)" => The cat sat on <a href="#learn-regex"><strong>cat</strong></a>.
</pre>
[Prueba la expresión regular](https://regex101.com/r/8Efx5G/1)
## 5. Banderas
Los indicadores también se llaman modificadores porque modifican la salida
de una expresión regular. Estos indicadores se pueden utilizar en cualquier orden
o combinación, y son una parte integral de RegExp.
|Bandera|Descripción|
|:----:|----|
|i|Insensible a mayúsculas y minúsculas: ajusta la coincidencia para que no distinga mayúsculas y minúsculas.|
|g|Búsqueda global: busque un patrón en toda la cadena de entrada.|
|m|Multilinea: Ancla meta caracter trabaja en cada linea.|
### 5.1 Mayúscula y minúscula
El modificador `i` se utiliza para realizar la coincidencia entre mayúsculas y
minúsculas. Por ejemplo, la expresión regular `/The/gi` significa: letra mayúscula
`T`, seguido del caracter en minúscula `h`, seguido del carácter `e`. Y al final
de la expresión regular, el indicador `i` indica al motor de expresiones
regulares que ignore el caso. Como puede ver, también ofrecemos el indicador
`g` porque queremos buscar el patrón en toda la cadena de entrada.
<pre>
"The" => <a href="#learn-regex"><strong>The</strong></a> fat cat sat on the mat.
</pre>
[Prueba la expresión regularn](https://regex101.com/r/dpQyf9/1)
<pre>
"/The/gi" => <a href="#learn-regex"><strong>The</strong></a> fat cat sat on <a href="#learn-regex"><strong>the</strong></a> mat.
</pre>
[Prueba la expresión regular](https://regex101.com/r/ahfiuh/1)
### 5.2 Búsqueda global
El modificador `g` se utiliza para realizar una coincidencia global
(encontrar todos las coincidencias en lugar de detenerse después de la primera coincidencia).
Por ejemplo, la expresión regular `/.(At)/g` significa: cualquier carácter,
excepto la nueva línea, seguido del caracter minúsculo `a`, seguido del caracter
en minúscula `t`. Debido a que siempre `g` prevee la bandera al final de la expresión
regular ahora encontrará todas las coincidencias de toda la cadena de entrada.
<pre>
"/.(at)/" => The <a href="#learn-regex"><strong>fat</strong></a> cat sat on the mat.
</pre>
[Prueba la expresión regular](https://regex101.com/r/jnk6gM/1)
<pre>
"/.(at)/g" => The <a href="#learn-regex"><strong>fat</strong></a> <a href="#learn-regex"><strong>cat</strong></a> <a href="#learn-regex"><strong>sat</strong></a> on the <a href="#learn-regex"><strong>mat</strong></a>.
</pre>
[Prueba la expresión regular](https://regex101.com/r/dO1nef/1)
### 5.3 Multilinea
El modificador `m` se utiliza para realizar una coincidencia de varias líneas.
Como analizamos anteriormente, las anclas `(^,$)` se utilizan para comprobar si
el patrón es el comienzo de la entrada o el final de la cadena de entrada. Pero
si queremos que las anclas funcionen en cada línea usamos la bandera `m`.
Por ejemplo, la expresión regular `/at(.)?$/Gm`
significa: caracter en minúscula` a`, seguido del caracter minúsculo `t`,
opcionalmente cualquier cosa menos la nueva línea. Y debido a `m` bandera ahora
el motor de expresión regular coincide con el patrón al final de cada línea de una cadena.
<pre>
"/.at(.)?$/" => The fat
cat sat
on the <a href="#learn-regex"><strong>mat.</strong></a>
</pre>
[Prueba la expresión regular](https://regex101.com/r/hoGMkP/1)
<pre>
"/.at(.)?$/gm" => The <a href="#learn-regex"><strong>fat</strong></a>
cat <a href="#learn-regex"><strong>sat</strong></a>
on the <a href="#learn-regex"><strong>mat.</strong></a>
</pre>
[Prueba la expresión regular](https://regex101.com/r/E88WE2/1)
## Bonus
* *Positive Integers*: `^\d+$`
* *Negative Integers*: `^-\d+$`
* *US Phone Number*: `^+?[\d\s]{3,}$`
* *US Phone with code*: `^+?[\d\s]+(?[\d\s]{10,}$`
* *Integers*: `^-?\d+$`
* *Username*: `^[\w.]{4,16}$`
* *Alpha-numeric characters*: `^[a-zA-Z0-9]*$`
* *Alpha-numeric characters with spaces*: `^[a-zA-Z0-9 ]*$`
* *Password*: `^(?=^.{6,}$)((?=.*[A-Za-z0-9])(?=.*[A-Z])(?=.*[a-z]))^.*$`
* *email*: `^([a-zA-Z0-9._%-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,})*$`
* *IPv4 address*: `^((?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?))*$`
* *Lowercase letters only*: `^([a-z])*$`
* *Uppercase letters only*: `^([A-Z])*$`
* *URL*: `^(((http|https|ftp):\/\/)?([[a-zA-Z0-9]\-\.])+(\.)([[a-zA-Z0-9]]){2,4}([[a-zA-Z0-9]\/+=%&_\.~?\-]*))*$`
* *VISA credit card numbers*: `^(4[0-9]{12}(?:[0-9]{3})?)*$`
* *Date (DD/MM/YYYY)*: `^(0?[1-9]|[12][0-9]|3[01])[- /.](0?[1-9]|1[012])[- /.](19|20)?[0-9]{2}$`
* *Date (MM/DD/YYYY)*: `^(0?[1-9]|1[012])[- /.](0?[1-9]|[12][0-9]|3[01])[- /.](19|20)?[0-9]{2}$`
* *Date (YYYY/MM/DD)*: `^(19|20)?[0-9]{2}[- /.](0?[1-9]|1[012])[- /.](0?[1-9]|[12][0-9]|3[01])$`
* *MasterCard credit card numbers*: `^(5[1-5][0-9]{14})*$`
* *Hashtags*: Including hashtags with preceding text (abc123#xyz456) or containing white spaces within square brackets (#[foo bar]) : `\S*#(?:\[[^\]]+\]|\S+)`
* *@mentions*: `\B@[a-z0-9_-]+`
## Contribution
* Report issues
* Open pull request with improvements
* Spread the word
* Reach out to me directly at ziishaned@gmail.com or [![Twitter URL](https://img.shields.io/twitter/url/https/twitter.com/ziishaned.svg?style=social&label=Follow%20%40ziishaned)](https://twitter.com/ziishaned)
## License
MIT © [Zeeshan Ahmed](mailto:ziishaned@gmail.com)

138
README.md
View File

@ -11,24 +11,24 @@
## What is Regular Expression? ## What is Regular Expression?
> Regular expression is a group of characters or symbols which is used to find a specific pattern from a text. > Regular expression is a group of characters or symbols which is used to find a specific pattern from a text.
A regular expression is a pattern that is matched against a subject string from left to right. The word "Regular expression" is a A regular expression is a pattern that is matched against a subject string from left to right. The word "Regular expression" is a
mouthful, you will usually find the term abbreviated as "regex" or "regexp". Regular expression is used for replacing a text within mouthful, you will usually find the term abbreviated as "regex" or "regexp". Regular expression is used for replacing a text within
a string, validating form, extract a substring from a string based upon a pattern match, and so much more. a string, validating form, extract a substring from a string based upon a pattern match, and so much more.
Imagine you are writing an application and you want to set the rules for when a user chooses their username. We want to 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 allow the username to contain letters, numbers, underscores and hyphens. We also want to limit the number of
characters in username so it does not look ugly. We use the following regular expression to validate a username: characters in username so it does not look ugly. We use the following regular expression to validate a username:
<br/><br/> <br/><br/>
<p align="center"> <p align="center">
<img src="https://i.imgur.com/ekFpQUg.png" alt="Regular expression"> <img src="https://i.imgur.com/ekFpQUg.png" alt="Regular expression">
</p> </p>
Above regular expression can accept the strings `john_doe`, `jo-hn_doe` and `john12_as`. It does not match `Jo` because that string Above regular expression can accept the strings `john_doe`, `jo-hn_doe` and `john12_as`. It does not match `Jo` because that string
contains uppercase letter and also it is too short. contains uppercase letter and also it is too short.
## Table of Contents ## Table of Contents
- [Basic Matchers](#1-basic-matchers) - [Basic Matchers](#1-basic-matchers)
- [Meta character](#2-meta-characters) - [Meta character](#2-meta-characters)
@ -60,8 +60,8 @@ contains uppercase letter and also it is too short.
## 1. Basic Matchers ## 1. Basic Matchers
A regular expression is just a pattern of characters that we use to perform search in a text. For example, the regular expression A regular expression is just a pattern of characters that we use to perform 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` means: the letter `t`, followed by the letter `h`, followed by the letter `e`.
<pre> <pre>
"the" => The fat cat sat on <a href="#learn-regex"><strong>the</strong></a> mat. "the" => The fat cat sat on <a href="#learn-regex"><strong>the</strong></a> mat.
@ -69,7 +69,7 @@ A regular expression is just a pattern of characters that we use to perform sear
[Test the regular expression](https://regex101.com/r/dmRygT/1) [Test the regular expression](https://regex101.com/r/dmRygT/1)
The regular expression `123` matches the string `123`. The regular expression is matched against an input string by comparing each 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 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`. case-sensitive so the regular expression `The` would not match the string `the`.
@ -81,8 +81,8 @@ case-sensitive so the regular expression `The` would not match the string `the`.
## 2. Meta Characters ## 2. Meta Characters
Meta characters are the building blocks of the regular expressions. Meta characters do not stand for themselves but instead are Meta characters are the building blocks of the 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. interpreted in some special way. Some meta characters have a special meaning and are written inside square brackets.
The meta characters are as follows: The meta characters are as follows:
|Meta character|Description| |Meta character|Description|
@ -114,8 +114,8 @@ letter `r`.
## 2.2 Character set ## 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 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 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`. expression `[Tt]he` means: an uppercase `T` or lowercase `t`, followed by the letter `h`, followed by the letter `e`.
<pre> <pre>
@ -134,8 +134,8 @@ A period inside a character set, however, means a literal period. The regular ex
### 2.2.1 Negated character set ### 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 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 character set. For example, the regular expression `[^c]ar` means: any character except `c`, followed by the character `a`, followed by
the letter `r`. the letter `r`.
<pre> <pre>
@ -146,13 +146,13 @@ the letter `r`.
## 2.3 Repetitions ## 2.3 Repetitions
Following meta characters `+`, `*` or `?` are used to specify how many times a subpattern can occur. These meta characters act Following meta characters `+`, `*` or `?` are used to specify how many times a subpattern can occur. These meta characters act
differently in different situations. differently in different situations.
### 2.3.1 The Star ### 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 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 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. character set. For example, the regular expression `[a-z]*` means: any number of lowercase letters in a row.
<pre> <pre>
@ -161,9 +161,9 @@ character set. For example, the regular expression `[a-z]*` means: any number of
[Test the regular expression](https://regex101.com/r/7m8me5/1) [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 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 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 spaces, followed by lowercase character `c`, followed by lowercase character `a`, followed by lowercase character `t`, followed by
zero or more spaces. zero or more spaces.
<pre> <pre>
@ -185,8 +185,8 @@ letter `c`, followed by at least one character, followed by the lowercase charac
### 2.3.3 The Question Mark ### 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 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 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`. character `h`, followed by the lowercase character `e`.
<pre> <pre>
@ -203,7 +203,7 @@ character `h`, followed by the lowercase character `e`.
## 2.4 Braces ## 2.4 Braces
In regular expression braces that are also called quantifiers are used to specify the number of times that a 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 ( 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). characters in the range of 0 to 9).
@ -213,7 +213,7 @@ characters in the range of 0 to 9).
[Test the regular expression](https://regex101.com/r/juM86s/1) [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 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. the comma the regular expression `[0-9]{3}` means: Match exactly 3 digits.
<pre> <pre>
@ -230,10 +230,10 @@ the comma the regular expression `[0-9]{3}` means: Match exactly 3 digits.
## 2.5 Character Group ## 2.5 Character Group
Character group is a group of sub-patterns that is written inside Parentheses `(...)`. As we discussed before that in regular expression Character group is a group of sub-patterns that is written inside Parentheses `(...)`. 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 character group then if we put a quantifier after a character then it will repeat the preceding character. But if we put quantifier after a character group then
it repeats the whole character group. For example, the regular expression `(ab)*` matches zero or more repetitions of the character "ab". it repeats the whole character 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 character group. For example, the regular expression `(c|g|p)ar` means: lowercase character `c`, We can also use the alternation `|` meta character inside character 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`. `g` or `p`, followed by character `a`, followed by character `r`.
<pre> <pre>
@ -244,10 +244,10 @@ We can also use the alternation `|` meta character inside character group. For e
## 2.6 Alternation ## 2.6 Alternation
In regular expression Vertical bar `|` is used to define alternation. Alternation is like a condition between multiple expressions. Now, In regular expression Vertical bar `|` is used to define alternation. Alternation is like a condition 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 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 is that character set works on character level but alternation works on expression level. For example, the regular expression
`(T|t)he|car` means: uppercase character `T` or lowercase `t`, followed by lowercase character `h`, followed by lowercase character `e` `(T|t)he|car` means: 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`. or lowercase character `c`, followed by lowercase character `a`, followed by lowercase character `r`.
<pre> <pre>
@ -272,17 +272,17 @@ expression `(f|c|m)at\.?` means: lowercase letter `f`, `c` or `m`, followed by l
## 2.8 Anchors ## 2.8 Anchors
In regular expressions, we use anchors to check if the matching symbol is the starting symbol or ending symbol of the 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 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 `$` that checks if matching character is the last character of the character of the input and the second type is Dollar `$` that checks if matching character is the last character of the
input string. input string.
### 2.8.1 Caret ### 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 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 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 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, 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`. followed by lowercase character `h`, followed by lowercase character `e`.
<pre> <pre>
@ -299,8 +299,8 @@ followed by lowercase character `h`, followed by lowercase character `e`.
### 2.8.2 Dollar ### 2.8.2 Dollar
Dollar `$` symbol is used to check if matching character is the last character of the input string. For example, regular expression Dollar `$` symbol is used to check if matching character is the last character of the input string. For example, regular expression
`(at\.)$` means: a lowercase character `a`, followed by lowercase character `t`, followed by a `.` character and the matcher `(at\.)$` means: a lowercase character `a`, followed by lowercase character `t`, followed by a `.` character and the matcher
must be end of the string. must be end of the string.
<pre> <pre>
@ -317,7 +317,7 @@ must be end of the string.
## 3. Shorthand Character Sets ## 3. Shorthand Character Sets
Regular expression provides shorthands for the commonly used character sets, which offer convenient shorthands for commonly used 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: regular expressions. The shorthand character sets are as follows:
|Shorthand|Description| |Shorthand|Description|
@ -332,10 +332,10 @@ regular expressions. The shorthand character sets are as follows:
## 4. Lookaround ## 4. Lookaround
Lookbehind and lookahead sometimes known as lookaround are specific type of ***non-capturing group*** (Use to match the pattern but not Lookbehind and lookahead sometimes known as lookaround are specific type of ***non-capturing group*** (Use to match the pattern but not
included in matching list). Lookaheads are used when we have the condition that this pattern is preceded or followed by another certain included in matching list). Lookaheads 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 $10.88`. pattern. For example, we want to get all numbers that are preceded by `$` character from the following input string `$4.44 and $10.88`.
We will use following regular expression `(?<=\$)[0-9\.]*` which means: get all the numbers which contain `.` character and are preceded We will use following regular expression `(?<=\$)[0-9\.]*` which means: get all the numbers which contain `.` character and are preceded
by `$` character. Following are the lookarounds that are used in regular expressions: by `$` character. Following are the lookarounds that are used in regular expressions:
|Symbol|Description| |Symbol|Description|
@ -348,11 +348,11 @@ by `$` character. Following are the lookarounds that are used in regular express
### 4.1 Positive Lookahead ### 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 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 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 equal sign is used like this: `(?=...)`. Lookahead expression is written after the equal sign inside those parentheses, a question mark with equal sign is used like this: `(?=...)`. Lookahead expression is written after the equal sign inside
parentheses. For example, the regular expression `[T|t]he(?=\sfat)` means: optionally match lowercase letter `t` or uppercase letter `T`, parentheses. For example, the regular expression `[T|t]he(?=\sfat)` means: optionally match lowercase letter `t` or uppercase letter `T`,
followed by letter `h`, followed by letter `e`. In parentheses we define positive lookahead which tells regular expression engine to match followed by letter `h`, followed by letter `e`. In parentheses we define positive lookahead which tells regular expression engine to match
`The` or `the` which are followed by the word `fat`. `The` or `the` which are followed by the word `fat`.
<pre> <pre>
"[T|t]he(?=\sfat)" => <a href="#learn-regex"><strong>The</strong></a> fat cat sat on the mat. "[T|t]he(?=\sfat)" => <a href="#learn-regex"><strong>The</strong></a> fat cat sat on the mat.
@ -362,9 +362,9 @@ followed by letter `h`, followed by letter `e`. In parentheses we define positiv
### 4.2 Negative Lookahead ### 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 Negative lookahead is used when we need to get all matches from input string that are not followed by a pattern. Negative lookahead
defined same as we define positive lookahead but the only difference is instead of equal `=` character we use negation `!` character 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 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. input string that are not followed by the word `fat` precedes by a space character.
<pre> <pre>
@ -375,8 +375,8 @@ input string that are not followed by the word `fat` precedes by a space charact
### 4.3 Positive Lookbehind ### 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 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 `(?<=...)`. 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`. are after the word `The` or `the`.
<pre> <pre>
@ -387,8 +387,8 @@ are after the word `The` or `the`.
### 4.4 Negative Lookbehind ### 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 Negative lookbehind is used to get all the matches that are not preceded by a specific pattern. Negative lookbehind is denoted by
`(?<!...)`. For example, the regular expression `(?<!(T|t)he\s)(cat)` means: get all `cat` words from input string that `(?<!...)`. For example, the regular expression `(?<!(T|t)he\s)(cat)` means: get all `cat` words from input string that
are not after the word `The` or `the`. are not after the word `The` or `the`.
<pre> <pre>
@ -399,7 +399,7 @@ are not after the word `The` or `the`.
## 5. Flags ## 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 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. combination, and are an integral part of the RegExp.
|Flag|Description| |Flag|Description|
@ -410,9 +410,9 @@ combination, and are an integral part of the RegExp.
### 5.1 Case Insensitive ### 5.1 Case Insensitive
The `i` modifier is used to perform case-insensitive matching. For example, the regular expression `/The/gi` means: uppercase letter 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 `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 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 whole input string.
<pre> <pre>
@ -429,9 +429,9 @@ the whole input string.
### 5.2 Global search ### 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 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 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 every matches from whole input character `t`. Because we provided `g` flag at the end of the regular expression now it will find every matches from whole input
string. string.
<pre> <pre>
@ -448,9 +448,9 @@ string.
### 5.3 Multiline ### 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 `m` modifier is used to perform a multi-line match. As we discussed earlier anchors `(^, $)` are used to check if pattern is
the beginning of the input or end of the input string. But if we want that anchors works on each line we use `m` flag. For example, the the beginning of the input or end of the input string. But if we want that anchors works on each line we use `m` flag. For example, the
regular expression `/at(.)?$/gm` means: lowercase character `a`, followed by lowercase character `t`, optionally anything except new 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. line. And because of `m` flag now regular expression engine matches pattern at the end of each line in a string.
<pre> <pre>
@ -496,7 +496,7 @@ line. And because of `m` flag now regular expression engine matches pattern at t
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