Ruby: Regular expression summary * Code sample available

We've organized the main regular expression patterns for beginners.

List of main regular expression patterns

pattern meaning
[a-z] Matches any one of the characters enclosed in square brackets (in this case matches any of the characters a through z)
\d Match numbers
\D Match other than numbers
{n} Matches the previous pattern that repeats n times
{n, m} Matches the previous character that appears at least n times and at most m times
. Matches any single character except newline
* Matches 0 or more repetitions of the previous subexpression
+ Matches one or more repetitions of the previous subexpression
\A The character string immediately after matches the character string at the beginning
\z The previous string matches the trailing string
?= Matches the character string immediately after the set character
*? Check the character string in which the character set immediately before continues 0 times or more,?Returns one character when the character immediately after


pattern meaning
/i Case insensitive

Deepen the meaning of "*"

pattern meaning
* Matches 0 or more repetitions of the previous character

Since it is a little difficult to understand "match 0 or more repetitions" of "*", check it by comparing with "+".

pattern meaning Examples of matching strings
a.*z A string of two or more digits starting with a and ending with z az, abz,abcz etc.
a.+z A string of 3 or more digits starting with a and ending with z abz,abcz etc.

Since "\ *" itself does not require a character string of one or more digits, a two-digit character string will match as in the above example. "." Matches any character except a newline, but with a "\ *" after it, it matches even if there is no character corresponding to ".". On the other hand, "+" asks for a character string with one or more digits, so it does not match a two-digit character string.

List of main patterns of regular expressions that can be used as they are

pattern meaning
/\A[Ah-Hmm-One-龥]/ Full-width hiragana, full-width katakana, kanji
/\A[A-Car-]+\z/ Full-width katakana
/\A[a-zA-Z0-9]+\z/ Half-width alphanumeric
/\A\d{3}[-]\d{4}\z/ Postal code("-And 7 digits)

Validation that can be used with Rails ActiveRecord function

Unlike regular expressions, Rails' ActiveRecord validation helper, numericity, allows you to easily configure validation with the following options:

pattern meaning
greater_than_or_equal_to: 〇〇 Numbers over 〇〇
less_than_or_equal_to: 〇〇 Numbers below 〇〇

A site where you can try regular expressions

There is a site called Rubular where you can try regular expressions that can be used in Ruby. It is a useful site to deepen your understanding of regular expressions because you can check whether the regular expressions you created match the intended character strings and patterns.

Reference material

Ruby 3.0.0 Reference Manual> Regular Expressions Rails Guide v6.0 Active Record Validation ( Basic regular expression list TechRacho [Series: Regular Expression] Unicode Character Properties (1)

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