Minimal basics on Regular Expression (Non-technical )

• replace all carbonate to polycarbonate
• Find all coders word in a file. It should not match aliencoders, decoders, and encoders. It should match only coders
• Replace nasty characters from Unix file which came from windows files like ^M
• Find all Cisco Pix Firewall from different firewall products

• It can be used anywhere, where we need to manipulate a text file, set of strings using a pattern. It is being used in almost all the programming languages and Perl is a well-known language whose power lies behind regex.
• It is being used in text editors, even in MS Word
• If you want to replace a word, say by mistake you wrote gray instead of grey and it is having more than 50 such occurrences in a file. Just write a pattern and use it. Ex: s/gray/grey/ig.  It will change all gray to grey in a fraction of seconds.
• You can save many lines by using regular expression rather than using nested if else.
• If it’s Perl then you can use code also, instead of just a string pattern. You need to use x modifier for that.
• One can use regular expression in grep and map function in Perl which can save many lines of codes ( an alternative to loop structure with if else conditions ). There are lot more instances.
• Regular expression is used by all level of programmers from different programming language domain like C#.NET, JavaScript, Perl, Python, PHP
• For Syntax highlighting, data validation using regex in your codes
• Even search engines like Google also uses Regular Expressions.

• If work can be done using simple if else or loop. Ex: do ip address validation (1-3 digit in four octet) and then use if else to check octets lies between 0-255 range or not.
• If you are using backtrack with greedy search, beware of using it. It may hang your system
• If you need to use regex anyhow , then don’t try to put all things in one complex regex
• It’s very powerful tool, if used wisely. Always try to use minimal greedy pattern like .*? instead of .*

• Character class: whatever you use inside [] will be treated as range with or options  i.e [a-z0-9A-Z] which means anything from a to z or A-Z or 0 to 9
• Meta characters (special characters) : character that has special meaning in regular expression other than simple meaning. Like + in regular expression world means one or more occurrence not a simple addition. other examples are \w,\d,\S,.,\t, ^, $, |

• Quantifier or range operators: It is used for defining range of the pattern like one or more, at least 2 or more, between 3 and 8 Ex:
  • \d{3,} means at least 3 digits or more  (ha){3} matches 3 ha i.e. hahaha (shortcut to laugh (he){3} -> hehehe 😛
  • \w +  means one or more occurrence of alphanumeric character including ‘_’
  •  Other quantifiers are ?, *, {} (Hint: greedy, non-greedy used for these)
• Modifier or (pattern modifier): used after that pattern to modify the working of regular expression
  • Ex: s/old/new/ig means substitute old to new wherever it finds the word old with case insensitive property
  • Other modifiers are i.e. m,i,c,x,g ,d,s,o  etc which depends upon regular expression operators. Like e modifier can be used only for substitute regex operator
• Saving the matched pattern using grouping pattern i.e. (\d+), if matched will be saved in $1
• Regular Expression operators: it defines that the pattern is for matching or searching or transliteration i.e. m, s, tr
• Other terms like greedy pattern, non-greedy pattern, backtrack etc. Like how .* and .*? works