Regular expression \b \b understand word boundary matching in simple terms

When using regular expressions , You will encounter situations where you need to accurately match certain characters , Now \b \B That comes in handy . If you don't understand these two regular expression metacharacters , It's not easy to use . This article describes how to use them .

1 \b \B The meaning of representation

Metacharacters	meaning
\b	Matches a word boundary
\B	Matches a nonword boundary
\w	Match a letter 、 Number or underscore . Equivalent to [A-Za-z0-9_]

The meaning of these two metacharacters looks very official , But it is very accurate . After understanding, you will find that they are concise and comprehensive , There is no nonsense , But you need to savor it carefully .

To understand these two metacharacters , Two concepts need to be clarified ：

1. word ： Speaking of words , It also involves a metacharacter \w, As shown in the table above ,\w Represents an alphanumeric or underscore . And words , It is by several \w Composed of , That is, any combination of letters and numbers .
2. The border ： In regular expressions , The boundary is not a specific character , But something that doesn't exist , A concept . Can be said to be \b \B Something that represents , The boundary is the thing between any two characters .

After clarifying these two concepts , The following can explain \b \B Represents the meaning of .

• \b Word boundaries ： Match a boundary , One side character is \w, One side character is not \w
• \B Non word boundary ： Matching elimination \b Outside the boundary . Both characters are \w, Or both characters are not \w

Here is the next definition , Express in regular expression language ：

• Character is a \w：\w
• Characters are not \w：[^\w]

As shown in the figure below , All imaginary vertical lines in the figure are boundaries .

Above picture , Each boundary satisfies ： On one side, \w, On one side, [^\w]. namely ：

• On the left is \w, On the right is [^\w]
• On the left is [^\w], On the right is \w

\B Except \b All boundaries beyond .\b It means that one side is \w, On one side, [^\w] The boundary of the , Then take the opposite , namely ：

• On both sides \w
• On both sides [^\w]

in short ：

Boundary type	The left character	Right hand character
\b	\w	[^\w]
\b	[^\w]	\w
\B	\w	\w
\B	[^\w]	[^\w]

The characters used are \w representative 1, Character is a [^\w] representative 0, Then understanding with the logical thinking of computer is ：

Boundary type	The left character	Right hand character
\b	1	0
\b	0	1
\B	1	1
\B	0	0

The representation of two binary numbers is also in these four cases . therefore , The following conclusions can be drawn ：

• \b Represents the boundary , The left and right character types are different
• \B Represents the boundary , The left and right character types are the same

Be careful ： There are only two types of characters （ yes \w 、 No \w）. Everything else can be forgotten , Just remember these two conclusions .

See this , You should be able to understand what word boundaries are from the chart above \b, What are non word boundaries \B 了 .

2 Usage examples

Here according to article 1 The conclusion drawn in section ：

• \b Represents the boundary , The left and right character types are different
• \B Represents the boundary , The left and right character types are the same

Analyze with several examples .

2.1 A simple example

2.1.1 Example 1

For mode \bat\b The first of \b, Because its right side is \w, Then its left side must be [^\w];
For mode \bat\b Second of \b, Because its left side is \w, Then its right side must be [^\w];
Therefore, the matching result is shown in the figure .

2.1.2 Example 2

For mode \Bat\b Medium \B, Because its right side is \w, Then the left side must also be \w;
For mode \Bat\b Medium \b, Because its left side is \w, Then its right side must be [^\w];
Therefore, the matching result is shown in the figure .

2.1.3 Example 3

For mode \Bat\B The first of \B, Because its right side is \w, Then the left side must also be \w;
For mode \Bat\B Second of \B, Because its left side is \w, Then its right side must also be \w;
Therefore, the matching result is shown in the figure .

2.1.4 Example 4

For mode \bat\B Medium \b, Because its right side is \w, Then its left side must be [^\w];
For mode \bat\B Medium \B, Because its left side is \w, Then its right side must also be \w;
Therefore, the matching result is shown in the figure .

2.2 Complex examples

If you can understand the simple example above , It is not difficult to understand the following .

2.2.1 Example 5

because % yes [^\w], Therefore, the characters on both sides of the matching result should also be [^\w]

Needless to say, this should be able to understand .

2.2.2 Example 6

According to two \B, Target thank Both sides should be \w

according to \b,r On the left should be [^\w]; according to \B,= On the right should be [^\w]

because r yes \w, according to \b,r On the left should be [^\w];
because = yes [^\w], according to \b,= On the right should be \w;
Two in the string regex== Are not satisfied with the left side is [^\w], The right side is \w, So the match failed .

The second in the string regex== Meet the left side is \w, The right side is \w, So the match was successful .