How do word boundaries work in regex?

13 Answers. A word boundary, in most regex dialects, is a position between \w and \W (non-word char), or at the beginning or end of a string if it begins or ends (respectively) with a word character ( [0-9A-Za-z_] ). So, in the string “-12” , it would match before the 1 or after the 2.

What is \b word boundary?

The metacharacter \b is an anchor like the caret and the dollar sign. It matches at a position that is called a “word boundary”. After the last character in the string, if the last character is a word character. Between two characters in the string, where one is a word character and the other is not a word character.

What are examples of word boundaries?

Some examples of phrases and sentences of how word boundaries are determined.

• As a /matter of fact/, I will /go away/ as soon as I finish my food.
• It is just because I have the /fear of God/ that I will allow them to travel in their order of merit.
• The leader of the team shared /butter and bread/ to his followers.

How do I capture a word in regex?

To run a “whole words only” search using a regular expression, simply place the word between two word boundaries, as we did with ‹ \bcat\b ›. The first ‹ \b › requires the ‹ c › to occur at the very start of the string, or after a nonword character.

What is a non-word character in regex?

Non-word characters include characters other than alphanumeric characters ( – , – and – ) and underscore (_). Task. You have a test string . Your task is to match the pattern. Here denotes any word character and denotes any non-word character.

What is considered a word boundary?

A word boundary is a zero-width test between two characters. For the word boundary test only, which must always have two characters to consider, the beginning and end of the string are considered non-word characters.

How do you identify word boundaries?

Tests of Word Identification

1. Potential pause: Say a sentence out loud, and ask someone to ‘repeat it very slowly, with pauses.
2. Indivisibility: Say a sentence out loud, and ask someone to ‘add extra words’ to it.
3. Phonetic boundaries: It is sometimes possible to tell from the sound of a word where it begins or ends.

How do you explain a word boundary?

Boundary, border, frontier share the sense of that which divides one entity or political unit from another. Boundary, in reference to a country, city, state, territory, or the like, most often designates a line on a map: boundaries are shown in red.

What are word characters in regex?

A word character is a member of any of the Unicode categories listed in the following table.

• Ll (Letter, Lowercase)
• Lu (Letter, Uppercase)
• Lt (Letter, Titlecase)
• Lo (Letter, Other)
• Lm (Letter, Modifier)
• Nd (Number, Decimal Digit)
• Pc (Punctuation, Connector)

When does a word boundary occur in regex?

A word boundary can occur in one of three positions: 1 Before the first character in the string, if the first character is a word character. 2 After the last character in the string, if the last character is a word character. 3 Between two characters in the string, where one is a word character and the other is not a word character.

When do you use a regular expression in regex?

Between two characters in the string, where one is a word character and the other is not a word character. Simply put: [&b&] allows you to perform a “whole words only” search using a regular expression in the form of [&bword&][&b&]. A “word character” is a character that can be used to form words.

Where does the regex match between two words?

Advancing a character and restarting with the first regex token, b matches between the space and the second i in the string. Continuing, the regex engine finds that i matches i and s matches s. Now, the engine tries to match the second b at the position before the l. This fails because this position is between two word characters.

Where do the start and end of word boundaries come from?

Therefore, the “start of word” and “end of word” boundaries derive their meaning from the \\b boundary. In non-Unicode mode, it matches a position where only one side is an ASCII letter, digit or underscore.