Regular expression to match characters at beginning of line only

Question

I am trying to work on regular expressions  I have a mainframe file which has several fields  I have a flat file parser which distinguishes several types of records based on the first three letters of every line  How do I write a regular expression where the first three letters are  CTR

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CTR  or  CTR    edit  To be more clear   CTR will match start of line and those chars  If all you want to do is match for a line itself  and already have the line to use   then that is all you really need  But if this is the case  you may be better off using a prefab substr   type function  I don t know  what language are you are using  But if you are trying to match and grab the line  you will need something like    or     or whatever  depending on what language regex function you are using

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There s are ambiguities in the question   What is your input string  Is it the entire file  Or is it 1 line at a time  Some of the answers are assuming the latter  I want to answer the former   What would you like to return from your regular expression  The fact that you want a true   false on whether a match was made  Or do you want to extract the entire line whose start begins with CTR  I ll answer you only want a true   false match   To do this  we just need to determine if the CTR occurs at either the start of a file  or immediately following a new line          n CTR

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CTR      matches a line starting with CTR

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Not sure how to apply that to your file on your server  but typically  the regex to match the beginning of a string would be     CTR    The   means beginning of string   line

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Try  CTR     which literally means start of line  CTR  anything   This will be case-sensitive  and setting non-case-sensitivity will depend on your programming language  or use   Cc  Tt  Rr     if cross-environment case-insensitivity matters

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Beginning of line or beginning of string    Start and end of string    CTR           delimiter     start of string CTR   literal CTR     end of string      zero or more of any character except newline    Start and end of line    CTR    m       delimiter     start of line CTR   literal CTR     end of line      zero or more of any character except newline m   enables multi-line mode  this sets regex to treat every line as a string  so   and   will match start and end of line    While in multi-line mode you can still match the start and end of the string with  A Z permanent anchors    ACTR   Z m    A   means start of string CTR   literal CTR      zero or more of any character except newline  Z   end of string m   enables multi-line mode   As such  another way to match the start of the line would be like this      A  r  n  r n CTR      or       r  n  r n CTR       r   carriage return   old Mac OS newline  n   line-feed   Unix Mac OS X newline  r n   windows newline    Note  if you are going to use the backslash   in some program string that supports escaping  like the php double quotation marks    then you need to escape them first  so to run  r nCTR   you would use it as    r  nCTR

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Regex symbol to match at beginning of a line       Add the string you re searching for  CTR  to the regex like this     CTR   Example  regex  That should be enough    However  if you need to get the text from the whole line in your language of choice  add a  match anything  pattern        CTR     Example  more regex  If you want to get crazy  use the end of line matcher       Add that to the growing regex pattern     CTR      Example  lets get crazy  Note  Depending on how and where you re using regex  you might have to use a multi-line modifier to get it to match multiple lines  There could be a whole discussion on the best strategy for picking lines out of a file to process them  and some of the strategies would require this    Multi-line flag m  this is specified in various ways in various languages contexts     CTR   gm   Example  we had to use m on regex101

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i     r n  CTR    i  -- case insensitive -- Remove if case sensitive     r n   -- ignore space and new lines   -- 0 or more times the same CTR - your starts with string

[regex] Regular expression to match characters at beginning of line only

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