Anchors

While literal characters, character classes, and the . match a single character, anchors don't match any character at all. Instead, they match a position before, after, or between characters.

• Anchors are therefore said to be zero-length.
• They are called anchors because they can be used to “anchor” the regex match at a certain position

^

^ matches the position before the first character in the string.

• Applying ^a to abc matches a.
• ^b does not match abc at all

$

$ matches right after the last character in the string.

• c$ matches c in abc
• a$ does not match at all.

\b

matches at a position that is called a “word boundary”

• \b allows you to perform a “whole words only” search using a regular expression in the form of \bword\b

\b is not technically an anchor, but it is very closely related. If we were to imagine a string of text as the space between 2 walls, then we would consider the anchors ^ and $ to be either wall (ie. the position before the first character, and after the last character). \b differs from this because \b marks the position of a word character \w on one side, and a non-word character \W on the other.

There are three different positions that qualify as word boundaries:

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.

Using ^ and $ as Start of Line and End of Line Anchors

Called “multi-line mode”, and it generally needs to be explicitly enabled.

If you have a string consisting of multiple lines, like first line\nsecond line, it often makes sense to think in terms of lines, rather than the entire string

• ex. ^ can match at the start of the string (ie. before the letter f), as well as after each line break (between \n and the letter s)
• ex. Likewise, $ still matches at the end of the string (after the last e), and also before every line break (between e and \n)

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Anchors are very important when using regex in a programming language to validate user input