Regular Expressions

Last updated on 2023-10-05 | Edit this page

Estimated time: 35 minutes



  • What are regular expressions?
  • Where and how can you use regular expressions in the MarcEditor?
  • What types of edits can you accomplish with regular expressions?


  • Understand what regular expressions are and how they can support advanced editing in the MarcEditor

Regular Expressions

Regular expressions, often referred to as regex, are a tool you can use to match, capture and manipulate data across your MARC record file. Regular expressions are comprised of a sequence of literal characters and metacharacters which are formulated to match a particular pattern found in your data that you want to manipulate. A detailed introduction to working with regular expressions can be found in the Library Carpentry lesson Introduction to Working with Data (Regular Expressions). In this lesson we will focus on the application of regex in the MarcEditor.

Regular Expressions in MarcEdit

MarcEdit uses the .NET regular expression syntax. Visit the .Net Regular Expression Language - Quick Reference for a breakdown of the syntax used for regex metacharacters in MarcEdit. You can also download the .Net Regex metacharacter reference sheet to support the creation of your own regular expressions in MarcEdit.

Many MarcEdit functions support the use of regular expressions for data selection and manipulation. They include: Find and Replace, Edit Field, Copy Field, Swap Field, Build New Field, Delete Field, Record Validation, and both the Extract and Delete Selected Records tool. The intent of this lesson is to demonstrate where and how you can leverage regular expressions to work with your MARC data.

Edit Field Data

With regular expressions you can easily locate variations in your data and replace them with a single value. In our file the OCLC identifier in the 035 field uses the prefixes on, ocn, and ocm, but our library formats the OCLC with the (OCoLC) prefix. Instead of running three Edit Field Data functions to update these prefixes, we can use a regular expression in the Edit Field Data tool to replace these prefixes with the (OCoLC) prefix in a single edit.

Update 035 OCLC prefixes

  1. Select Tools → Edit Field Data
  2. Enter the following values: Field: 035 Find: on|ocn|ocm Replace: (OCoLC)
  3. Check Use Regular Expressions box
  4. Click drop down menu → Preview Results
  5. Verify your results look as expected, close preview tab and then click Process In this regular expression we use the pipe (|) metacharacter to specify that we are looking for the string on OR ocn OR ocm. We then replace the captured string with (OCoLC).

Knowing your dataset

Regular expressions are a powerful tool, but using them can sometimes lead to unexpected and undesirable results if you do not know your dataset. In the Edit Field Data exercise above, any 035 field containing the string on, ocn, or ocm would have had these values replaced with (OCoLC).

For example: =035 \\$a(SFUonline)638023 would have become =035 \\$a(SFU(OCoLC)line)638023

Reviewing your dataset before employing regular expressions is a good best practice. The MarcEditor Preview Results feature also supports trial and error.

Find and Replace

The Find and Replace tool is another useful way to identify and manipulate data in the MarcEditor with the support of regular expressions. In our file the local call numbers in the 090 field are formatted without a space between the class and subclass (ex. LD1780 and not LD 1780). To update the call number to follow our local policy we need to isolate the components of the call number so we can add a space between them. We can do this with a regular expression by employing groups.

Add a space between 090 class and subclass

  1. Select Edit → Find
  2. Enter =090 in the Find box and click Find All Review the list of 090 fields, what observations can you make about the call numbers in this field? Review shows us that not all call numbers begin with letters. We do not want to add a space to these call numbers and need to account for this in our regular expression.
  3. In the Find window, click Replace to bring up the Replace Text functions
  4. In the Find box enter the regular expression (=090 \\\\\$a[A-Z]+)(\d.*) This regular expression uses two sets of () to capture groups and then manipulate them.
  • Broken down, the first group (=090 \\\\\$a[A-Z]+) will match the literal string =090 \\$a (the additional backslashes are used to escape the regex metacharacters \ and $ so that they will be read as literals) followed by any single capital letter in the range A to Z ([A-Z]) one or more times (+). By specifying that the regular expression must locate a capital letter at the start of the call number, the regular expression will not edit the call numbers that begin with a digit.
  • The second group (\d.*) will match any digit (\d) followed by any character (.) zero or more times (*)
  1. In the Replace box enter $1 $2 This regular expression refers back to the groups we defined and captured in the Find box. $1 refers to the group defined in the first set of brackets, and $2 refers to the group defined in the second set of brackets. If we had defined additional groups they would be referred to chronologically as $3, $4 etc. The regular expression $1 $2 will output the contents of the two captured groups with a space between them.
  2. Check the Use regular expressions box
  3. Click drop down menu → Preview Results
  4. Verify your results look as expected, close preview tab and then click Process

Select Records for Edit

The Select Records for Edit tool, introduced in lesson 6, allows you to isolate and then work with a subset of your records. Regular expressions can be a powerful tool to help you isolate the subset of records you want to work with.

In this example we want to add the electronic journals genre form (655 \\4$aElectronic journals) to our electronic continuing resource records.

Working with fixed fields, where character position and coded values impart specific meaning, can be difficult for the human eye. However, regular expressions make it easy to specify the position of a character in a text string in order to find, select or update that data.

Isolate continuing resource records

  1. From the top level menu select File → Select Records for Edit
  2. The file you are working on will be selected by default in the Source MARC file box. To import the Leader for review, enter LDR in the Display Field Box and Click Import.
  3. To isolate continuing resource records we find any records where LDR/07 = i or s. In the Find box, enter the regular expression .{7}[is].{16}
  • This regular expression looks for any 7 characters (.{7})(Leader positions 0-6), followed by the character i OR s ([is]) (Leader position 7), followed by any 16 characters (.{16})(Leader positions 8-23)
  1. Check the Use Regular Expressions box and then click the magnifying glass. A popup window will let you know how many records were selected based on your criteria.
  2. Click Export Selected. You will get a popup will let you know the records have been extracted. Click OK.

Inverting your selection

When using the Select Records for Edit tool you can use the Invert Selections option to select all records that DID NOT meet your criteria. This option can be helpful when you want to select all records except for those that meet a specific criteria.

Edit selected records

Now that we’ve isolated our continuing resource records, we can add a field for the electronic journals genre form (655 \4$aElectronic journals) to the electronic records in our subset.

  1. From the top level menu select Tools → Add/Delete Fields
  2. In the Batch Editing window, enter 655 in the Field box and enter \4$aElectronic journals in the Field Data Box. Under General Options select Use Regular Expression, and under Add Field Options select Add Field If Present/NOT Present.
  3. To add our 655 field to electronic records only, we need to specify what should be present in the record for the new field to be added. What are some fields and values we might look at to determine format? For this example we are going to use a regular expression to look for the presence of an 856 link or a the carrier type online resource. Enter the regular expression =856|=338.+online resource in the Find What box.
  • This regular expression looks for the string =856 OR the string =338 followed by any character (.) one or more times (+), followed by the string online resource
  1. Click Add Field. Review your data, were the new fields added correctly?

After editing your subset of records, you can either save the edits back into the original file, or save the subset of records to a new file.

To save the edits back into the original file, select File → Save (Ctrl+S). You will get a dialogue box indicating that your records have been saved back into the original file.

To save your subset of records separately, select File → Save As. Specify the name and location of your new file and click Ok.

Key Points

  • Regular expressions are a powerful tool for selecting and editing your data in the MarcEditor
  • Regular expressions can be used in many functions and tools in MarcEdit and the MarcEditor
  • Knowing your data is essential to ensuring your regular expressions are working as intended