regex - Regular expression to match characters at beginning of line only

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Answer 1

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 permanent anchors

/ACTR.*/m

A = means start of string
CTR = literal CTR
.* = zero or more of any character except newline
= end of string
m = enables multi-line mode

As such, another way to match the start of the line would be like this:

/(A|
|
|
)CTR.*/

or

/(^|
|
|
)CTR.*/

= carriage return / old Mac OS newline
= line-feed / Unix/Mac OS X newline
= 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 CTR.* you would use it as "\r\nCTR.*"