lessons.regex101.title

Or it works like. It waits for one of the expressions it reserved to match.

Alternation

Asterisk

Matches the beginning of the string or line.

Start of string or line

Ignore Case

Character Set

Digit

End of string or line

Matches any character except line breaks.

Ensures that the expression does not stop on the first match.

Global

Group

If not enabled, line start and end equals the beginning and end of the entire string. It doesn't work for each row individually.

Multiline

Negated Character Set

Negative Lookahead

Negative Lookbehind

Makes a grouping that cannot be referenced.

Non Capturing Group

Not Digit

Not Whitespace

Not Word

Matches a word character or position that is not at the end of a word.

Not Word Boundary

Optional

Plus

Positive Lookahead

Positive Lookbehind

Expression matches within specified ranges.

Quantifier

Matches all characters between two characters, including themselves.

Range

Reference

Explore all Regex patterns and symbols with the Regex cheatsheet.

Regex Cheatsheet

Whitespace

Matches any alphanumeric character. Including the underline.

Word

Matches the word character or position at the end of a word.

Word Boundary

Use the asterisk `*` to write the expression that will select each word where the letter `e` is absent or is present one or more times.

Practice: Asterisk `*`

Write the expression that will select the words `of` in the text.

Practice: Basic Matcher

Write the phrase that matches each word in the text. The only characters that change are the initials of the words.

Practice: Character Sets

Type the expression to select individual letters, numbers, spaces, and special characters in the text. The expression you type must match any character.

Practice: Any Character

Practice completed!

Write down the expression that will match anything other than the words `beor` and `beur` in the text. Do this using the negated character set.

Practice: Negated Character Sets

Write the expression using the plus sign `+` to select words in which the letter `e` occurs one or more times in the text.

Practice: Plus Sign `+`

Write the expression using curly braces `{}` that will find texts containing `4` numbers side by side. Remember that the range `[0-9]` will match a single digit.

Practice: Curly Braces - 1

Write the expression using curly braces `{}` that will find texts containing at least `2` numbers side by side.

Practice: Curly Braces - 2

Write the expression using curly braces `{}` that will find texts containing at least `1` and at most `4` numbers side by side.

Practice: Curly Braces - 3

Write the expression indicating that the letter `n` is optional in the text, using the question mark `?`. Thus, both the words `a` and `an` can be selected.

Practice: Question Mark `?`

Write the expression that will select the letters from `g` to `k` in the text.\n`g` and `k` letters should also be included in this range.

Practice: Letter Range

Type an expression to select numbers from `2` to `7` in the text.\n`2` and `7` should also be included in this range.

Practice: Number Range

Now it's time to do some practice to consolidate what we've learned.

Practice

Learning Regex is easier than you think. You can use this tool to easily `learn`, `practice` and `test` Regex.

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Learn Regex step by step, from zero to advanced.

In regex, the period `.` can match anything, including spaces. But this match is only for a single character where it is used. The asterisk character `*` means that the preceding item does not exist at all or occurs more than once in the text. When the two are used together, it means that there can be an element of unlimited length where used.\n\nBelow are various keywords. Add `.*` to the end of the regex to write the expression that starts with `how to write` and matches everything after it.

Anything `.*`

Let's take a look at some Regexes that you will use frequently. We recommend that you review the `Regex 101` tutorial to learn more about these. Type `Yes` to get started.

Basics

Let's say we have a list of URLs, and we want to find only those URLs that start with `http`. But URLs also contain `http` in different parts other than their beginning. So add a caret `^` to the beginning of the following expression to filter out only those starting with `http`.

Caret Sign `^`

To filter the text that contains the words or phrases we're looking for, we write the word we're looking for between our two phrases `.*` that match everything. For example, type `.*buy.*` to filter out what includes `buy`.

Contains

Again we have a list of URLs. We want to find only those URLs ending in `.htm`.\n\nSince the period `.` is a special character that matches everything, we first need to disable the period before `htm` using the escape character `\`. This applies to all special characters.\n\nThen add a `$` sign to the end of the expression. This ensures that the text to be filtered ends with the expression written before it.

Dollar Sign `$` and Escape Character `\`

Here is an example of using regex in the `.htaccess` file. In the example, `php` extension links are directed to `html` extensions with `301` code.

.htaccess

We may want to filter the results by character lengths. For example, to filter URLs that are `35` characters long, type the number you want to use for the length, `{35}`, using curly braces at the end of the period `.`.

Length Restrictions `{n}`

To filter URLs with a maximum length of `35` characters, type `{1,35}` the minimum and maximum number values we will use for the length, using curly braces at the end of the period `.`.

Length Restrictions (Maximum) `{n,m}`

To filter URLs with a minimum length of `35` characters, type `{35,}` using curly braces at the end of the period and ending with a comma for the minimum number value we will use for the length.

Length Restrictions (Minumum) `{n,}`

We use the negated character sets `[^]` to denote characters we don't want to be included. The characters we do not want to be included are written in square brackets with a caret at the beginning. Below are both `http` and `https`. Since `http` occurs in both, it will also choose `https`. Type `[^s]` after the letter `p` to exclude the letter `s`.

Negated Character Sets `[^abc]`

We use it to express that an expression or character is optional. For example, we want to filter text containing both `http` and `https`. We can use the Or operator `|` for this operation, but there is a better way for these two almost identical words.\n\nIn this example, the letter `s` is optional. Instead, add a question mark `?` to the end of the letter `s`.

Optional Operator `?`

We use the Or operator `|` to filter out multiple expressions. For example, filter text containing `seo` and `regex` by writing the two words, separating them with the Or operator `|`.

Or Operator `|`

Regex knowledge is a skill that increases productivity on any platform that allows you to use regex. Once you start using it and improve this skill, it will help you complete many repetitive tasks faster and save time for other things. To better understand the basics of Regex, you can visit the `Regex 101` tutorial. You can use the `Playground` page to test your expressions on your texts.

Outro

Here is an example of regex-like usage in the `robots.txt` file. The example states that bots should not crawl links with the extension `pdf`.

robots.txt (regex-like syntax)

Regex is a set of regular expressions frequently used in many fields, especially by people who want to increase their productivity. The advantages of using Regex for SEO are that it makes it easier to analyze big data and write richer filters. We recommend starting with the `Regex 101` tutorial if you are new to Regex. \n\nStart by typing `OK` in the Regex field to proceed to the first step and access more detailed explanations.

Regex for SEO

You can use Regex for SEO on platforms such as `Google Analytics`, `Google Data Studio`, `Google Sheets`, `Google Search Console`, `Ahrefs`, `Deepcrawl`, `Screaming Frog`. It is also frequently used in `.htaccess` and `robots.txt (regex-like syntax)` files.\n\nBelow is a sample regex that will select only texts in the list that start with `Google`.

Use Cases

You can learn the basics of Regex in this tutorial.

Regex 101 - EN

In this tutorial, you can learn how and where you can use Regex for SEO.

Regex for SEO - EN

Regex Learn - Not Found

Sometimes you just need to remember Regex. Try the cheatsheet that will make it easier for you to understand and remember better.

Regex Learn - Regex Cheatsheet

Learn Regex interactively, practice at your level, and test your own Regex.

Regex Learn - Step by step, from zero to advanced.

Learn Regex step by step. Complete the interactive tutorial that will make your learning easier and faster.

Regex Learn - Regex Interactive Course

You can easily build and test your regex pattern online via Playground.

Regex Learn - Playground

With `RegexLearn Cheatsheet`, you can see all of the usages you need at once and check them directly from the one page. Explore all Regex patterns and symbols with the Regex cheat sheet.

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Cheatsheet

`Regular Expressions`, abbreviated as Regex or Regexp, are a string of characters created within the framework of Regex syntax rules. You can easily manage your data with Regex, which uses commands like finding, matching, and editing. Regex can be used in programming languages such as Python, SQL, JavaScript, R, Google Analytics, Google Data Studio, and throughout the coding process. Learn regex online with examples and tutorials on RegexLearn now.

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Learn

Regex Learn is an open-source project that welcomes community contributions and is free to use. Working on this project allows you to hone your skills, study, and collaborate.  You can contribute & support here.

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Open Source

If you want to generate a Regex, you can easily build, validate and test your Regex pattern in the Playground.

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Playground

If you complete the learning part, it's time to PRACTICE! In the practice section, you can test your Regex knowledge and practice with Regex tutorials based on your level. Soon.

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We put an asterisk `*` after a character to indicate that the character may either not match at all or can match many times. For example, indicate that the letter `e` should never occur in the text, or it can occur once or more side by side.

Asterisk `*`

The character or word we want to find is written directly. It is similar to a normal search process. For example, to find the word `curious` in the text, type the same.

Basic Matchers

We were using `[0-9]` to find numbers. To find only numbers at the beginning of a line, prefix this expression with the `^` sign.

Caret Sign `^`:\nSelecting by Line Start

If one of the characters in a word can be various characters, we write it in square brackets `[]` with all alternative characters. For example, to write an expression that can find all the words in the text, type the characters `a`, `e`, `i`, `o`, `u` adjacently within square brackets `[]`.

Character Sets `[abc]`

You can return to the previous steps whenever you want, and you can easily navigate through all the steps you have passed.

Congratulations, you have completed all the steps!

Let's use the `$` sign after the `html` value to find the `html` texts only at the end of the line.

Dollar Sign `$`:\nSelecting by End of Line

The period `.` allows selecting any character, including special characters and spaces. Type a period `.` in the Regex field to proceed.

Dot `.`: Any Character

There are special characters that we use when writing regex. `{ } [ ] / \ + * . $^ | ?` Before we can select these characters themselves, we need to use an escape character `\`. For example, to select the dot `.` and asterisk `*` characters in the text, let's add an escape character `\` before them.

Escape Character `\`

Flags change the output of the expression. That's why flags are also called `modifiers`. Flags determine whether the typed expression treats text as separate lines, is case sensitive, or finds all matches. Continue to the next step to learn the flags.

Flags

In order to remove the case-sensitivity of the expression we have written, we must activate the `case-insensitive` flag.

Case-insensitive Flag

The `global` flag causes the expression to select all matches. If not used it will only select the first match. Now enable the `global` flag to be able to select all matches.

Global Flag

Regex sees all text as one line. But we use the `multiline` flag to handle each line separately. In this way, the expressions we write to identify patterns at the end of lines work separately for each line. Now enable the `multiline` flag to find all matches.

Multiline Flag

Regex does a greedy match by default. This means that the matching will be as long as possible. Check out the example below. It refers to any match that ends in `r` and can be any character preceded by it. But it does not stop at the first match.

Greedy Matching

Below, we have two groups: `(ha)` and `(haa)`. When we want to reference these groups later in the pattern, we use `\1` for the first group and `\2` for the second group. For example, in the pattern `(ha)-\1,(haa)-\2`, the `-` is outside the groups, and `\1` refers to `ha` while `\2` refers to `haa`. Type `\2` at the end of the expression to refer to the second group.

Referencing a Group

We can group an expression and use these groups to reference or enforce some rules. To group an expression, we enclose `()` in parentheses. For now just group `haa` below.

Parentheses `( )`: Grouping

Lazy matching, unlike greedy matching, stops at the first matching. For example, in the example below, add a `?` after `*` to find the first match that ends with the letter `r` and is preceded by any character. It means that this match will stop at the first letter `r`.

Lazy Matching

If we want the phrase we're writing to come before or after another phrase, we need to "lookaround". Take the next step to learn how to "lookaround".

Lookarounds

To find all words in the text below, except for `ber` and `bor`, type `e` and `o` side by side after the caret `^` character inside square brackets `[]`.

For example, we want to select numbers other than the hour value in the text. Therefore, we need to write the negative look-ahead `(?!)` expression after our expression to select only the numerical values that do not have `PM` after them. Include `PM` after the `!` sign inside the parentheses.

Negative Lookahead: `(?!)`

For example, we want to select numbers in the text other than the price value. Therefore, to select only numeric values that are not preceded by `$`, we need to write the negative lookbehind `(?&lt;!)` before our expression. Add `\$` after the `!` inside the parenthesis.

Negative Lookbehind: `(?&lt;!)`

Adding `?:` right after the opening parenthesis, like `(?:ha)`, makes a group non-capturing: it still groups the expression, but its match is not stored, so it cannot be referenced. Below, the first group `(?:ha)` is non-capturing. That is why the reference `\1` refers to the capturing group `(haa)`.

Parentheses `(?: )`: Non-capturing Grouping

`\d` is used to find only number characters.

Number Character `\d`

It allows to specify that an expression can be in different expressions. Thus, all possible statements are written separated by the pipe sign `|`. This differs from charset `[abc]`, charsets operate at the character level. Alternatives are at the expression level. For example, the following expression would select both `cat` and `rat`. Add another pipe sign `|` to the end of the expression and type `dog` so that all words are selected.

Pipe Character `|`

To indicate that a character must occur one or more times, we put a plus sign `+` after a character. For example, indicate that the letter `e` must occur one or more times in the text.

Plus Sign `+`

For example, we want to select the hour value in the text. Therefore, to select only the numerical values that have `PM` after them, we need to write the positive look-ahead expression `(?=)` after our expression. Include `PM` after the `=` sign inside the parentheses.

Positive Lookahead: `(?=)`

For example, we want to select the price value in the text. Therefore, to select only the number values that are preceded by `$`, we need to write the positive lookbehind expression `(?<=)` before our expression. Add `\$` after the `=` sign inside the parenthesis.

Positive Lookbehind: `(?<=)`

To express a certain number of occurrences of a character,  at the end we write curly braces `{n}` along with how many times we want it to occur. For example, indicate that the following letter `e` can occur only `2` times in a row.

Curly Braces - 1

To express at least a certain number of occurrences of a character, immediately after the character we write at least how many times we want it to occur in a row followed by a comma `,` and wrapped inside curly braces `{n,}`. For example, indicate that the following letter `e` can occur at least `3` times in a row.

Curly Braces - 2

To express the occurrence of a character in a certain number range, we write curly braces `{x,y}` for the inclusive interval. For example, indicate that the following letter `e` can only occur between `1` and `3` times in a row.

Curly Braces - 3

To indicate that a character is optional, we put a `?` question mark after a character. For example, indicate that the following letter `u` is optional.

Question Mark `?`

To find the letters in the specified range, the starting letter and the ending letter are written in square brackets `[]` with a dash between them `-`. It is case-sensitive. Type the expression that will select all lowercase letters between `e` and `o`, including themselves.

Letter Range`[a-z]`

To find the numbers in the specified range, the starting number and the ending number are written in square brackets `[]` with a dash `-` between them. Write an expression that will select all numbers between `3` and `6`, including themselves.

Number Range`[0-9]`

Some special characters are used to specify how many times a character will be repeated in the text. These special characters are the plus `+`, the asterisk `*`, and the question mark `?`.

Repetitions

`\s` is used to find only space characters.

Space Character `\s`

Regex is short for `Regular Expression`. It helps to match, find or manage text. The lessons and the Playground use JavaScript's regex engine, where a pattern is written as `/pattern/flags`. Start by typing `OK` in the Regex field to proceed to the first step and access the more detailed description.

Intro

Regular Expressions are a string of characters that express a search pattern. Often abbreviated as `Regex` or `Regexp`. It is especially used to find or replace words in texts. In addition, we can test whether a text complies with the rules we set.\n\n For example, let's say you have a list of filenames. And you only want to find files with the `pdf` extension. Following typing an expression `^\w+\.pdf$` will work. The meaning of the definitions in this expression will become clearer as the steps progress.

What is Regular Expressions `Regex`?

`\D` is used to find non-numeric characters.

Except Number Character `\D`

`\S` is used to find non-space characters.

Except Space Character `\S`

The expression `\W` is used to find characters other than letters, numbers, and underscores.

Except Word Character `\W`

The expression `\w` is used to find letters, numbers and underscore characters. Let's use the expression `\w` to find word characters in the text.