A Quantifier Inside A Lookbehind Makes It Non Fixed Width7 min read

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a quantifier inside a lookbehind makes it non fixed width

In regular expressions, a quantifier specifies how many times a particular pattern should be matched. Quantifiers can be either fixed width or non-fixed width.

A fixed width quantifier matches a specific number of characters, while a non-fixed width quantifier matches any number of characters up to a certain point. For example, the regular expression \d+ matches one or more digits, while the regular expression .+ matches any number of characters.

Inside a lookbehind, a quantifier can only be fixed width. This means that the lookbehind will only match a specific number of characters. If you try to use a non-fixed width quantifier, it will be ignored.

For example, the regular expression (?<=\d+)+ matches one or more digits, while the regular expression (?<=\.+)+ matches any number of characters including a period.

What is a lookbehind in regex?

A lookbehind is a regex feature that allows you to check whether a certain pattern exists at the beginning of a string, without having to scan the entire string for the pattern.

The lookbehind is specified using the (?<=) syntax. For example, the following regex will match the word "cat" only if it appears at the beginning of a string:

cat(?<=)

The lookbehind is limited to fixed-length strings. That is, you can only use a lookbehind to check for a pattern at the beginning of a string, not at the end.

What is a positive lookahead?

A positive lookahead is a programming construct that allows a programmer to specify a condition that must be met in order for a certain piece of code to execute. This condition is checked before the code is executed, and if it is not met, the code is not executed. This can be useful for ensuring that certain code is only executed under certain circumstances, or for ensuring that certain code is not executed if certain conditions are not met.

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What is quantifier in regex?

When you’re working with regular expressions, you may come across the term "quantifier." But what is a quantifier, and what do they do?

Simply put, a quantifier is a way of telling a regular expression engine how many times to match a particular pattern. There are four basic quantifiers: *, +, ?, and {n,m}.

The asterisk (*) is the most basic quantifier. It tells the engine to match the pattern as many times as possible. So, for example, the expression "a*b" would match "a", "ab", "aab", and so on.

The plus sign (+) is similar to the asterisk, but it tells the engine to match the pattern at least n times. For example, the expression "a+b" would match "a", "aa", "aaa", and so on.

The question mark (?) is the most versatile quantifier. It tells the engine to match the pattern either once or zero times. So, for example, the expression "a?b" would match "a" or "b".

Finally, the {n,m} quantifier tells the engine to match the pattern at least n times, but no more than m times. For example, the expression "a{2,3}b" would match "aab", "aazzb", and so on, but not "aaab".

What is lookahead and Lookbehind?

Lookahead and lookbehind are two features of regular expressions that allow you to check for certain patterns of characters at the beginning and end of a string, without having to match the entire string.

Lookahead lets you check for a pattern at the beginning of a string, without having to match the entire string. Lookbehind lets you check for a pattern at the end of a string, without having to match the entire string.

Both lookahead and lookbehind are optional features of regular expressions. They can be turned on or off with the (?=) and (?<=) metacharacters, respectively.

The following example shows how lookahead can be used to check for a pattern at the beginning of a string.

The regular expression (?=^\d+) matches a string that begins with a number.

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The following example shows how lookbehind can be used to check for a pattern at the end of a string.

The regular expression (?<=\d+) matches a string that ends with a number.

What does *$ mean in regex?

When using regular expressions, there are a few special characters that have a specific meaning. The asterisk (*) is one of these characters. When used in a regular expression, the asterisk means "match zero or more of the preceding character".

For example, if you want to match any string of letters, you could use the regular expression "a*". This would match any string that begins with the letter "a".

The dollar sign ($) is another special character in regular expressions. When used, it means "match the end of the string". For example, if you wanted to match the word "apple", you could use the regular expression "apple$. This would match the word "apple" as well as the string "apples".

What is non-capturing group in regex?

A regular expression (regex) is a sequence of characters that helps you match or find text in a string. Non-capturing groups are one of the several features that make regex powerful.

A non-capturing group is a special type of group that doesn’t capture the text it matches. This means that the text isn’t stored in a variable, and it’s not returned as part of the match. Non-capturing groups are useful when you need to group parts of a regex but don’t want to capture the text.

For example, say you want to find all the URLs in a string. You could use a regular expression like this:

/https?:\/\/[a-z0-9]+\.[a-z]{2,4}\//

This regex matches URLs that start with http or https, and it includes the domain name and extension. The group between the parentheses (https?:\/\/[a-z0-9]+\.[a-z]{2,4}\/) is a non-capturing group. This group doesn’t capture the text it matches, and it’s not returned as part of the match.

You can use non-capturing groups to combine multiple regexes into a single expression. For example, say you want to find all the URLs and email addresses in a string. You could use a regex like this:

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/(https?:\/\/[a-z0-9]+\.[a-z]{2,4}\/|[a-z0-9][email protected][a-z0-9]+\.[a-z]{2,4})/

This regex matches URLs and email addresses that start with http or https, or that have a domain name and extension, or that have an email address format. The group between the parentheses (https?:\/\/[a-z0-9]+\.[a-z]{2,4}\/|[a-z0-9][email protected][a-z0-9]+\.[a-z]{2,4}/) is a non-capturing group. This group doesn’t capture the text it matches, and it’s not returned as part of the match.

You can also use a non-capturing group to create a regex that’s easier to read and understand. For example, say you want to find all the URLs in a string, but you don’t want to include the domain name and extension. You could use a regex like this:

/(https?:\/\/[a-z0-9]+)\/

This regex matches URLs that start with http or https, and it doesn’t include the domain name and extension. The group between the parentheses (https?:\/\/[a-z0-9]+)\/) is a non-capturing group. This group doesn’t capture the text it matches, and it’s not returned as part of the match.

Non-capturing groups are a powerful tool that can help you match text more easily and accurately.

What is negative Lookbehind regex?

A regular expression (regex) is a special text string used to identify a particular pattern of characters in a text string. A regex can be used to search for and/or replace text strings.

One of the features of regular expressions is that they can include "lookbehind assertions". A lookbehind assertion is a special construct that allows you to check whether a particular pattern exists at the beginning of a text string, before the first character.

A "negative" lookbehind assertion is a lookbehind assertion that checks for the absence of a particular pattern at the beginning of a text string.

The "negative" in "negative lookbehind assertion" refers to the fact that the regex is looking for the absence of a pattern, rather than the presence of a pattern.

The "positive" lookbehind assertion is the more common type of lookbehind assertion. A positive lookbehind assertion checks for the presence of a particular pattern at the beginning of a text string.