Basic RegEx

Posted by Joy Lin on August 28, 2019

This blog will display the general rules of RegEx. There are some notations may different among different progamming languages. please refer to that particular languages for the details.

Why I would like to write about this topic?

I have seen so many applications/shell commands that need to use RegEx to produce the needed information. However, I know that I still can use other way to get the needed infomration. In bottom of my mind, sooner or later, if I would like to have clean codes in some area, (for example, validate the email address, phone number, IP address… etc.) RegEx would be a better choices to make the code lean and clean. The last section (Bonus section) of my first week curriculum is RegEx. This is a good oppertunity for me to dive deep into this topic, and make myself be familiar with it. I know I will be benifit from it in the near future. I also like to use this blog to track how I learn this, what the issues that I encountered, and etc…

What is RegEx? What is pos and con to use RegEx?

RegEx stands for “Regular Expression”.

How to form a pattern? (any good practice that can be followed?)

  • Match Literal String

    1. exact match => this is limitted to only one pattern Search a word in a string, paragraph, aritical, etc… this is the basic idea behind RegEx. Explicitly to spell out the searched key word surrounded by //. Note: the case of searched key impack the search result.

    2. with Different Possibilities => this can be used to search for multiple patterns using the alternation or OR orperator :
    3. Ignore the serached keyword case by using flag - i flag Note: the flag(s) is appended // note: you can append multiple flags after //. For example /search_key_word/ig
    4. wild card
      • . (dot or period) => use this wild card to match all the characters
    5. Match single character with multiple possibilities by using character classes => this allow you to define a group of characters you wish to match by placing them inside square brackes ([]) Note: inside the character classes, you can use hyphen (-) to define a range of characters to match, and includes both ends
      1. Match single characters not speicifed by using negated character sets => place (^) after the opening square bracket and before the characters you do not want to match
      2. Match characters that occur one or more times => use + to check it
  • flag:
    • i : ignore the case of the searched keyword
    • g : for global search meaning it will match all occurrences in the RegEx

      Is the same pattern can be used by all programming languages to generate the same results?

RegEx cheat sheet => please follow the following link: