Expression to find a specific length string surrounded by constants

Question

I do Regular Expressions so rarely that they always challenge me. Even simple ones.

How can I make a regular expression that will match all of these:

 := 'abc'
 := 'xyz'
 := '2rs'
 := 'abe'
 := 'a2c'

Basically it starts with := ' and ends with ' and has three values inside. Could be numbers or chars.

Answer 1

Something like this should work (as seen on rubular.com):

:= '([a-z0-9]{3})'

Explanation:

• := ' is matched literally since they're not metacharacters
• [a-z0-9] defines a character class matching lowercase letters and digits
• {3} is exact repetition, 3 times
• (...) is a capturing group (not needed, but probably handy)

Minor variations on this pattern include:

• [a-zA-Z0-9] instead to also allow uppercase letters
• {1,3} instead to allow between 1-3 repetition
• := *' instead to allow any number of space (* here means "zero or more repetition of")

regular-expressions.info

• Character Classes, Repetition, Brackets for Grouping
• Flavor comparison - has information about differences between major regex flavors