Matching only the first occurrence in a line with Regex

Question

I am completely new to regex and I would greatly appreciate any help.

The task is simple. I have a CSV file with records that read like this:

12345,67890,12345,67890
12345,67890,12345,67890
12345,67890,12345,67890
12345,67890,12345,67890
12345,67890,12345,67890

I would like to replace the first comma with a space and leave the rest of the commas intact, for every line. Is there a regex expression that will only match the first comma?

I tried this: ^.....,. This matches the comma, however, it also matches the entire length of the string preceding the comma, so if I try to replace this with a space all of the numbers are deleted as well.

Answer 1

The matching pattern could be:

^([^,]+),

That means

^        starts with
[^,]     anything but a comma
+        repeated one or more times (use * (means zero or more) if the first field can be empty)
([^,]+)  remember that part
,        followed by a comma

In e.g. perl, the whole match and replace would look like:

s/^([^,]+),/\1 /

The replacement part just takes the whole thing that matched and replaces it with the first block you remembered and appends a space. The coma is "dropped" because it's not in the first capturing group.

Answer 2

s/,/ /

This, by default (i.e. without the g option), replaces only the first match.

Answer 3

This should match only the first number and the comma: ^(\d{5}),. If you'd like to gobble up everything else in the line, change the regex to this: ^(\d{5}),(.*)$

Answer 4

More elegant solution is to use lazy matching:

s/^(.+?),/\1 /

that will group characters by moving from start of the string (^) towards the end by one character (.+?) on each step untill it finds first comma sign. All this group along with the first comma occurence will be replaces by group (\1) and space character.

Answer 5

TextPad always had the ability to use posix notation, but you have to change the settings in a different dialog box. To use TextPad's default settings for regular expressions, you have to "escape" the opening and closing parentheses:

Replace space after 5-digit zip code, at the beginning of each line

^\([0-9]+\)[ ]

With tab

\1\t

As above, the ^ means start of line

\( is an "escaped parenthesis" and it marks the beginning of the first search expression, i.e., the five digits

[0-9]+ means one or more digits (not just 5-digit zip codes)

\) is another "escaped parenthesis" to mark the end of the first search expression

[ ] is just a space character (you could leave out the brackets, but then no one would be able to see it on this web page :-)

In the replacement expression

\1 is the first search expression, the part between parentheses above (one or more digits)

\t is a tab character

So the search and replace command looks for one or more digits, followed by a space. Then it replaces all that with the same group of digits followed by a tab.

I don't think there is any way simply to find "a space that comes after 5 digits" so you can just replace the space without touching the digits. You have to find the 5 digits (the first string) followed by the space (the second string). Then, although it seems redundant or cumbersome, REPLACE the original string of 5 digits with ITSELF, followed by the tab (the second string).

Everyone who knows this forgets that newbies have no idea about this. That's why I'm spelling it out for you, my friend.

Ed Poor Math Tutor and retired Computer Programmer New York City

Answer 6

To match only the first occurrence of any regex expression remove all flags. Each regex expression comes with the following possible flags and typically defaults to using the global flag which will match more than one occurrence:

• /g = With this flag the search looks for all matches, without it – only the first match is returned
• /i = case insensitive
• /m = multi line mode
• /s = all . to match newline character \n
• /u = unicode
• /y = sticky mode (search in specific location)