Let's learn a little about Regular Expressions...

[Deepa Srivastav]

Hi Friends ,

It's my first blog and it may seem to be a small topic to write about but its surely an important concept in coding. I have seen quite a lot of posts (in community) and usage in Service Now .

I hope this blog helps you to understand about Regular expression

A regular expression is a special text/string for describing a search pattern. Use regular expressions to search through large numbers of text and binary files and then you can replace texts, rename files, copy files etc.

Meaning of different character/symbols:

• Twelve characters have special meanings in regular expressions: the backslash \, the caret ^, the dollar sign $, the period or dot ., the vertical bar or pipe symbol |, the question mark ?, the asterisk or star *, the plus sign +, the opening parenthesis (, the closing parenthesis ), the opening square bracket [, and the opening curly brace {. These special characters are often called "metacharacters". Most of them will produce are errors when used alone.
• If you want to use any of these characters as a literal in a regex, you need to escape them with a backslash. So, if you want to match 2+3=5, the correct regex is 2\+3=5.

I would not mention all the concepts present in regular expression but will try to put all the important one's here.

• Anchors do not match any characters. They match a position. ^ matches at the start of the string, and $ matches at the end of the string. Example:^l matches only first l in lol.
• A character class [ ] specify a range of characters, We can use more than one range inside brackets. A caret after the opening square bracket negates the character class.

          Example: [0-9a-z] matches only 0-9 digits and alphabets in (smaller case).

• Use curly braces { } to specify a repetition .Example: [A-Z]{2} two characters, each of which can be between A-Z.

Shorthand character classes:

• \d matches a single character that is a digit.
• \w matches a "word character" (alphanumeric characters plus underscore).
• \s matches a whitespace character.
• The dot matches a single character. Example: te.t matches text, test, tent etc.
• |   is regular expression equivalent of "or".

To test if a particular regex matches (part of) a string, you can call the string match() or test() method.

Example:   if(regexp.test(someValue)){}

Note: Regex engines are case sensitive by default.

I found below link very useful for learning regex. It lets you write the pattern and shows whether its correct or wrong.

http://regexone.com/

Some community posts for similar topic which will help you understand more and solve your issue (if you have the same).

https://community.servicenow.com/thread/223016?start=0&tstart=0
https://community.servicenow.com/message/951790#951790
https://community.servicenow.com/message/930850#930850
https://community.servicenow.com/message/947083#947083
https://community.servicenow.com/thread/232272
https://community.servicenow.com/thread/232690

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