Regular expressions per character

In general, regular expressions work per character so

• z matches one single 'z' character
• . (the dot) matches one single character - any character except newline
• [abc] (a character class) matches one single character, a or b or c
• \s matches a single "white space" character
• \p{...} matches a single character in the appropriate Unicode character class
• etc.

*, +, ? and {} are quantifiers that tell how many times the character that is on the left hand site can match:

• z* means 0 or more 'z' characters
• z+ means 1 or more 'z' characters
• z? means 0 or 1 'z' character.
• z{2, 4} means 2, 3 or 4 'z' characters.
• [abc]+ means matching a, b or c one or more times.

Then we can have sub-expressins enclosed in parentheses, and we can apply quantifiers to these subexpressions. So we can have a subexpression like #\d{2,4}-\d{4,7}\s+ and if we would like to have more of these matching one after the other, we can enclose it in () parentheses and put a quantifier (e.g. {3,}) after it: (#\d{2,4}-\d{4,7}\s+){3,} That quantifier will be applied to the whole subexpression, meaning that subexpression has to match 3 or more times.

This seems to match "words" or "strings", but in reality those "words" or "strings" are matched on the individual character level.

Regex cheat sheet

There are a number of articles about regexes you might want to read. The central one might be the regex cheat sheet that has links to all the other articles.

Comments

i don't understand meaning of [^a-z0-9]*[ \t\n\r][^a-z0-9]* .

--- What do you understand from this?