Simple Introduction to Regular expressions



Simple Introduction to Regular expressions

Regular expressions have a lot of application in normal programming, and for anyone who wants to learn programming, regular expressions are a must-have knowledge.

Not much nonsense to say, the following first to do a simple introduction to the regular expression, in the subsequent articles, will be detailed introduction.

One, meta-character

Metacharacters a total of 12: $ () [{? + *. ^ \ |

Metacharacters has a special meaning, and if you want to use its literal value, you must escape it.

such as: \$ \* \ (et cetera )

two, control characters or non-printable characters

\a Alarm

\e exit

\f Page Change

\ nthe line break

\ r Enter

\ t Horizontal Tabulation

\v Vertical Tabulation

Third, shorthand

\d single digit [0-9] ( Note: [] means one of all the characters in the brackets;-is a concatenated character, representing all characters greater than or equal to 0 less than or equal to 9 )

\w letters, numbers, underscores

\s whitespace characters, including spaces, line breaks, tabs

Note: \d \w \s matches characters that are not matched by \d \w \s

Iv. Repetition

Fixed number of times: {8}---repeated 8 times

Infinite times: {n}

0 or more times: {0,} equivalent to *

One or more times: {1,} equivalent to +

0 or one time: {0,1} equivalent to?

Let's look at an example:

The matching time format requires the following:

1. The hours and seconds are two digits, and the milliseconds are represented by three digits.

2. The corresponding range: 00--99

3. The range of minute and second corresponds: 00--59

4. Milliseconds corresponding to the range: 000--999

5. Format as follows: 12:34:56.789

Answer:

1, when the two digits are 0 to 9 one of them, we can be expressed as [0-9][0-9] or [0-9]{2} or \d\d or \d{2}

2, minute and second format is the same, and it is preceded by:, so we represent (: [0-5]\d) {2}

3, milliseconds can be expressed as \d\d\d or \d{3}

Total: \d\d (: [0-5]\d) {2}\.\d{3} note there is a meta-character ".", we take its literal value and need to be escaped.

Verify with Regexbuddy:



Simple Introduction to Regular expressions